Manchester United full back Rafael da Silva missed training on Monday morning with a thigh problem ahead of Tuesday's Champions League quarter-final clash with Bayern Munich.
David Moyes is already without his first choice left back in Patrice Evra who is suspended for the first leg clash against Pep Guardiola's side and could be forced to shuffle his defence further with Rafael now an uncertainty, the Guardian report.
Midfielder Tom Cleverley was also absent from the session, although Antonio Valencia, Chris Smalling and Phil Jones took part after recovering from respective injuries.
Rafael was shown a red card in his last encounter with Bayern when they eliminated United on away goals at the quarter final stage in 2010, although the 22-year-old won praise from former manager Sir Alex Ferguson for his tireless performance.
With Alexander Buttner expected to deputise in the absence of Evra, Valencia, Smalling and Jones will be the candidates to replace Rafael should he not recover in time. The Brazilian was to be handed the job of trying to shackle Franck Ribery at Old Trafford but that responsibility may now be passed onto to Jones, who has started his last six games for the club at centre half.
United's chances of overcoming the challenge presented by the current Champions League holders and recently crowned Bundesliga champions for a second consecutive year have been widely dismissed given their disastrous Premier League campaign.
However, United have retained their 100 per cent record at Old Trafford in European competition this season and while Jones is realistic of his side's chances, he is optimistic ahead of tomorrow's first leg clash.
"We will study them, I am sure, see what they are good at and try to exploit any weaknesses they have," Jones told United's official website.
"Listen, we are realistic and know how difficult the game is going to be. We know how well they are playing, they are in form and have won the league with seven games to go but who knows?
"It is the Champions League, it is a home tie at Old Trafford and if we get the crowd behind us then you just never know."