The France international was absent on Saturday along with team-mates Victor Lindelof and Luke Shaw
Scott McTominay’s header in the fifth minute against the Hornets was the game’s only goal at Old Trafford, sending United through to the fourth round with a 1-0 win.
But in his post-game press conference just days later, Solskjaer confirmed that all three were injured – though he did not elaborate on the nature of their concerns.
Asked about the trio, Solskjaer would only say: “They’re injured.”
With those three players out and Solskjaer looking to rotate his squad amid a busy fixture schedule, several fringe players got a chance to start against Watford.
Among them was Donny van de Beek, who hasn’t enjoyed a consistent run of playing time after making a summer move from Ajax.
Solskjaer was happy with what he saw from the Dutch international on Saturday, saying he demonstrated his considerable ability.
“I think Donny showed again his qualities, his pace, he is very technically clean,” Solskjaer said.
“One little flick to play Juan [Mata] through and he’s got that quality to open up tight games. He will benefit from this game as well.”
Brandon Williams also got a start against Watford. After a breakthrough campaign last season, Williams has seen his playing time limited in 2020-21, leading to talk he could leave the club on loan this month.
“I thought he had a very good game,” Solskjaer said of the 20-year-old defender. “He’s not played for a while, playing on right-hand side. It might be easier on the right side but very pleased with Brandon, one of the positives.”
United face a quick turnaround after their victory over Watford, as they travel to Turf Moor to face Burnley in a Premier League match on Tuesday.
Solskjaer’s side are currently level on points with Liverpool at the top of the table, and have played one game fewer than Jurgen Klopp’s men.