Ronaldo named in squad for Villarreal clash
A statement on the Premier League champion's website said Ronaldo had trained well on Monday morning after sustaining a leg injury and joined the 20-man squad for the flight to Spain.
However United said Gary Neville, Dimitar Berbatov, Edwin Van der Sar and Danny Welbeck would play no part in Tuesday's match.
Defender Neville is still nursing a groin strain and is unlikely to play in Sunday's Premier League derby match against Manchester City.
Berbatov has an injured hamstring while his fellow-striker Welbeck took no part in Monday's training session. Van der Sar has been rested.
Midfielder Paul Scholes, who has suffered knee ligament damage, trained on Monday but is not yet ready to play.
Defending champions United only need a point against Villarreal to qualify for the knockout stages.