Caolan Lavery could be set for a spell on the sidelines after suffering what manager Chris Wilder described as a 'big smash to the cheekbone' against Derby on Saturday.
Lavery was substituted in United's 3-1 win over Derby after clashing heads with Marcus Olsson as they both jumped for a header. Both players received lengthy treatment, and Lavery was substituted shortly after.
Lavery later posted a picture on Snapchat showing the damage, with his eye heavily bruised. United are next in action on September 9, when they travel to Sunderland.
Substitute Samir Carruthers was later replaced himself after picking up an injury against Derby, and Wilder said: "I want us to be competitive, I don't want that taken away from us, but we're not a cynical, dirty side.
"It was a 50/50 ball in the air, both boys went for it and unfortunately, Caolan came out with a big smash to his cheekbone, so we'll assess that.
"Samir got a big whack to the calf, and couldn't carry on. So that was disappointing for those two boys."