Gareth Southgate says England's situation is "more complicated than any other country" after announcing a 33-man provisional squad for Euro 2020.
The England manager must name a 26-man squad by 1 June.
Manchester United, Manchester City and Chelsea play European finals this week.
"There are 12 players still to play so we're always going to need additional players and added to that we have some injuries at different stages, that we have very little info about," he said.
"We felt more time will help us make better decisions. Our preference was to name the 26, but we have not got an ideal hand of cards – a lot of unknowns.
"Info and evidence are very important and we will have a lot more in the next seven days."
These are some of the key issues facing Southgate before 1 June – and at the bottom of the page you can pick your final 26-man squad.
- White and Godfrey among uncapped quartet in 33-man provisional England squad
Who will recover from injury in time?
While Burnley goalkeeper Nick Pope has missed out through injury, there remain fitness issues surrounding several Three Lions players.
Liverpool's Jordan Henderson, Everton's Dominic Calvert-Lewin and Leeds United's Kalvin Phillips also have injury concerns – though Leeds are confident Phillips can recover from his shoulder injury in time.
Meanwhile, Manchester United's Harry Maguire is battling to be ready in time for facing Villarreal in Wednesday's Europa League final.
"We have got a bit of a shortage of information on a couple of the injuries," Southgate told BBC Radio 5 Live.
"Understandably, Manchester United have a big game tomorrow so I get why we haven't been able to access that as easily. I don't know whether he is fit for tomorrow or in three months' time. That is where we are with it."
And Southgate will hope no other England stars pick up knocks as Manchester City face Chelsea in the Champions League final on Saturday, which could feature the likes of John Stones, Kyle Walker, Reece James, Ben Chilwell, Mason Mount and Phil Foden.
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How many right-backs can you take?
There is room for Liverpool's Trent Alexander-Arnold despite being left out of the previous England squad in March.
The 22-year-old is one of four right-backs in the provisional squad, along with Walker, James and Atletico Madrid's Kieran Trippier.
"You're looking at four right-backs, I just see four good footballers," said Southgate.
"Trippier can play at left-back. Alexander-Arnold can play at wing-back, I think he can play midfield. I've seen James play on the right side of a back three, at wing-back and as a midfielder. Those flexible players who can fulfil different roles will be hugely important."t
On Alexander-Arnold, he added: "Trent is very much in contention. People tend to think I don't think Trent is a good footballer. That just isn't the case.
"I think there is no doubt he could [play in midfield]. He's a fantastic passer of the ball and he has great passing vision. So he's a playmaker from right-back, that's kind of what he is."
Walker can also play at centre-back – as he has previously for England – but will Southgate really be able to fit all four into his final squad?
How much faith can he put in youth?
In the centre-back positions, Brighton's Ben White and Everton's Ben Godfrey are included among four uncapped players in the squad.
"They are both exciting young defenders," Southgate said. "They are flexible, they play in different roles, this is a great opportunity to get to know them a bit better and for them to work with the team.
"There is no way of knowing how long they will be with us at this stage but it should be a fantastic experience for them."
The other two uncapped players are goalkeepers Sam Johnstone, of West Brom, and Sheffield United's Aaron Ramsdale, who were both relegated from the Premier League this season.
Southgate's squad is bursting with young talent, including three teenagers in 17-year-old Borussia Dortmund midfielder Jude Bellingham, plus Arsenal's Bukayo Saka and Manchester United forward Mason Greenwood, who are both 19.
Eighteen of the 33 players have won fewer than 10 caps.
"There are too many question marks today," said Southgate. "It will become far clearer. We have got good contingencies in place – a nice balance with young players. If they're with us for one week or five weeks, it will be a brilliant experience.
"We're building towards a hugely exciting summer. We have to build a team who can win."
Is there a decision he might regret?
One experienced player who will miss out is Tottenham midfielder Eric Dier, who played at Euro 2016 and the 2018 World Cup.
"Unfortunately I just don't think his season has been long enough to be in the squad and that is a difficult call because I know how much he brings to the whole group," added the England manager.
"When you are at a tournament the players who put the team first are critical. It wasn't an easy conversation but he is an outstanding professional and took the news in a professional way."
Along with Everton defender Michael Keane, another notable absentee was Leicester's creative midfielder James Maddison.
"It's an area of the pitch where we have unbelievable competition for places," Southgate said.
"We feel there are players ahead of James in that area of the pitch."
With captain Harry Kane a guaranteed starter, there is also no space for Patrick Bamford – despite scoring 17 Premier League goals this season – or Southampton's Danny Ings in England's attacking department.
Could transfer talk distract Kane?
Recent reports indicate England captain Kane, 27, has asked to leave Tottenham – with some suggesting he wants to move before Euro 2020.
Kane is England men's sixth all-time top scorer with 34 goals and has said he hopes to have "a good, honest conversation" with Spurs chairman Daniel Levy about his future.
"I don't need to talk about Harry's focus. He has got one goal with us and that's to win a European Championship," said Southgate.
"What is going on at his club is a matter for him and Tottenham. He has made some statements in the last few days but that is out of the way now and I think it is very unlikely transfer deals will be done while we are away.
"We are not going to stop phones ringing and cannot control conversations going on in private. Most deals are done after tournaments."
Analysis – 'Pickford will be undisputed first choice'
BBC Sport chief football writer Phil McNulty
Southgate's decision to exclude experienced duo Dier and Keane drives at questions still to be answered in central defence, with Manchester United captain Maguire expected to be fit but still a slight concern after his ankle ligament injury.
Everton's Godfrey is a deserved inclusion after a superb season in which he has been under Southgate's constant scrutiny. Brighton's White also gets in but there is disappointment for Burnley's James Tarkowski, the excellent Aston Villa defender Ezri Konsa and Fikayo Tomori, who has full international experience and has flourished on loan at AC Milan from Chelsea.
The exclusion of Leicester City's Maddison is no surprise given his indifferent form and the emergence of so many talented players in that area. Southgate has never appeared convinced and this might just be the confirmation.
Southampton's Ings will feel hard done by but Southgate has great faith in the natural finishing ability of Greenwood. Leeds United's Bamford will also feel disappointed to not even make the provisional squad after a top-class campaign back in the Premier League.
And for all the questions earlier this season, Everton's Jordan Pickford will be undisputed first-choice keeper, thanks to a very good second half of the season, plus the unfortunate injury to Pope. Southgate is a huge Pickford fan and will be delighted he recovered his best form in time for the Euros.
Pick your final England Euros squad
Pick your 26-man England squad for the Euros.
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England's provisional 33-man squad in full
Goalkeepers: Dean Henderson (Man Utd), Sam Johnstone (West Brom), Jordan Pickford (Everton), Aaron Ramsdale (Sheff Utd)
Defenders: John Stones (Man City), Luke Shaw (Man Utd), Harry Maguire (Man Utd), Trent Alexander-Arnold (Liverpool), Kyle Walker (Man City), Tyrone Mings (Aston Villa), Reece James (Chelsea), Conor Coady (Wolves), Ben Chilwell (Chelsea), Ben Godfrey (Everton), Kieran Trippier (Atletico Madrid), Ben White (Brighton)
Midfielders: Mason Mount (Chelsea), Declan Rice (West Ham), Jordan Henderson (Liverpool), Jude Bellingham (Borussia Dortmund), Kalvin Phillips (Leeds), Jesse Lingard (Man Utd), James Ward-Prowse (Southampton)
Forwards: Harry Kane (Tottenham), Marcus Rashford (Man Utd), Raheem Sterling (Man City), Dominic Calvert-Lewin (Everton), Phil Foden (Man City), Jack Grealish (Aston Villa), Mason Greenwood (Man Utd), Ollie Watkins (Aston Villa), Jadon Sancho (Borussia Dortmund), Bukayo Saka (Arsenal)
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