The 20-year-old was forced off with a knee injury during last week's first leg, which ruled him out of Saturday's Premier League win over Fulham.
Concern: Rafael was taken off on a stretcher during the first leg clash with Chelsea
Manager Sir Alex Ferguson had stated the problem was merely bruisingrather than ligament damage, a fact confirmed by his presence on a sidepitch at United's Carrington training complex this morning (Monday) .
Ready for action: United are bidding to repeat their Treble success of 1999
However, as the defender was restricted to running rather than ball work with the rest of the group, whether he will be included in Ferguson's side remains open to question, especially as both John O'Shea and Wes Brown are available to play at right-back.
Ferguson said: 'Rafael did a bit of jogging this morning. Hopefully he will be able to be ready for tomorrow but I am not sure about it.
'There is a little doubt and it is not something I want to be risking in a major game.
'He really has to do something special tomorrow morning to change my mind about it.'
Brotherly bother: Fabio, unlike his twin, is available to play against Chelsea
With the exception of Scotland skipper Darren Fletcher, who has been laid low by a virus, the remainder of Ferguson's first-team squad were training.
That included Rio Ferdinand, who also missed Saturday's game afterreturning from a calf injury at Stamford Bridge, and Wayne Rooney whowas suspended.
Advantage: United are favourites to progress thanks to Wayne Rooney's goal at Stamford Bridge
However, Rooney is available as United look to protect the one goal lead given to them by the England international last week and book a semi-final place almost certainly against German challengers Schalke.
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Explore more:People: Wayne Rooney, Alex Ferguson, Darren Fletcher, Rio Ferdinand, Wes Brown, John O'Shea Places: Scotland, United Kingdom