Fletcher a doubt for Sunderland second leg
The striker was restricted to less than 20 minutes in Saturday's 2-2 draw with Southampton, coming on as a substitute for Jozy Altidore.
Gus Poyet's men head into the encounter at Old Trafford with a slender 2-1 advantage from the first leg, but face the prospect of being without the Scot for the return match as Sunderland attempt to reach Wembley.
"We were trying not to play him at all (against Southampton) because he had a problem, a little injury," Poyet said.
"We would not have involved him if we had been doing well.
"I asked him to take a risk for him and for his team-mates, and credit to him, he did.
"It's a tricky injury, and he could have made it worse, but he was happy in his warm-up and he gave us a very good 20 minutes."
Sunderland go in search of a first League Cup final appearance since losing 1-0 to Norwich City in the 1985 showpiece in high spirits, having lost just once in their last 10 matches in all competitions.