Sir Alex Ferguson is unsure whether Rio Ferdinand can cope with playing in all of England's matches at Euro 2012.
The Manchester United boss fears the 33-year-old's long-standing lower back problem would leave him unable to deal with the tough match schedule.
Rio Ferdinand Factfile
Makes his England debut against Cameroon eight days after turning 19
Joins Manchester United in £30m deal, breaking the British transfer record for a second time
Captains United to Champions League final victory over Chelsea
Wins his fifth Premier League title with United
England could play six matches in 21 days if they were to reach the final in Poland and Ukraine this summer.
Ferguson told MUTV: "You play something like a game every four days. Rio Ferdinand couldn't do that."
New England boss Roy Hodgson is set to name his squad on Wednesday.
He has yet to speak with Ferdinand and deposed skipper John Terry about their willingness to play alongside each other.
Fabio Capello took a gamble on Ledley King's fitness by taking him to the World Cup in 2010, only for the move to backfire when the defender's knee problems flared up again.
Ferdinand's stamina this season
Plays three full games in eight days - Swansea, Benfica, Newcastle
Appears in three games in eight days - Aston Villa (subbed on 64 mins), FC Basle, Wolves
Plays three full games in eight days - QPR, Wigan, Aston Villa
Ferdinand's back problem has led to calf, knee and hamstring injuries in the past, with the veteran making 37 appearances for United this season.
Ferguson has already written to the Football Association to explain why
only Ryan Giggs is available for Great Britain's football squad
at this summer's London Olympics.
Jonny Evans requires an ankle operation and
Chris Smalling may need groin surgery
, while the club insist that any players involved with Euro 2012 should not be considered for the Olympics.