OGS confirms McTominay will have a scan tomorrow.
United's front three of Rashford, Anthony Martial and Daniel James were too good for Brighton while 19-year-old left-back Brandon Williams, making his first Premier League start, was given a standing ovation by the Old Trafford crowd after being substituted in the final minutes.
The tie finished 3-1 with Ole Gunnar Solskjaer's side up to 7 in the Premier League, just one point adrift of 5 place Sheffield United.
The 22-year-old Scotland worldwide had earlier forced an own goal from Brighton's Davy Propper to put United 2-0 up, with goals from Andreas Pereira and Marcus Rashford eventually helping United up to seventh in the table.
"He's done nothing wrong since he's come in", said Solskjaer. Whatever game we're playing in, we go on the pitch to do our best and, of course, we're always looking at the league table.
The list of Manchester United's absentees shouldn't change much from the squad that featured Thursday against Partizan.
"He's really, really impressed me, I have to say". As a person, I don't doubt it and I'm delighted for him.
The only real negative on Sunday afternoon was the sight of Scott McTominay in pain after suffering an ankle injury.
McTominay heading down the tunnel on a stretcher. McTominay was able to walk out of Old Trafford, albeit with a bit of a limp, and the severity of the issue will be assessed on Monday.
Following the match, Ole Gunnar Solskjaer confirmed: "He [McTominay] went over on his ankle". "It's a painful one". I had one and was out for eight weeks but sometimes back in two.
The former Shakhtar Donetsk man has his eye on further cup success, but also identifies the need for United to be more consistent in the Premier League, starting on Sunday.
"They were the better team, deserved to win".
"We tried but we didn't play as well, the level we can play at, but we have to credit the opposition as well, so a bit of both".