Rio Ferdinand looks set to miss Tuesday night's Premiership encounter with Sheffield United after limping out of Saturday's FA Cup semi final win over Watford.

Ferdinand was originally a doubt for the game against the Hornets after picking up a groin strain in Tuesday night's Champions League clash. But, after aggravating the injury at Villa Park, Sir Alex revealed the United defender would undergo a medical scan on Sunday morning.

"Hopefully it's not a long-term injury," the United boss told MUTV. "We have three days' rest now so we'll take advantage of that."

Ferdinand was replaced in the 40th minute by Darren Fletcher, who made the unlikely move to right back. With Fletcher's introduction, Evra switched to his favoured left back position, while Heinze moved into the centre to partner Wes Brown.

Rio now joins fellow United defenders Gary Neville (ankle), Mikael Silvestre (shoulder), John O'Shea (calf) and Nemanja Vidic (shoulder) on the treatment table, although O'Shea and Neville are expected to return soon.

Sir Alex does have youngster Craig Cathcart available and could hand the 18-year-old his United debut when Neil Warnock's Sheffield United side arrive at Old Trafford on Tuesday