"He just had cramp. We were very conscious with him having been out for six weeks, we knew he wouldn't last 90 minutes," the Boro boss told the club's official website.
"We hoped he could give us an hour and we got 55 out of him in the end.
"He got through so much running in the first half that both he and Tuncay were almost bound to flag.
"That was an important goal for him. He needed that first goal for us because he's been torturing defences without getting the goal he deserves.
"For him to come to Old Trafford and score against the best defence in the country gives him and us confidence for future games."