Ole Gunnar Solskjaer praised Alexis Sanchez's attitude and challenged the forward "to come good" when he returns to full fitness.
Sanchez has struggled for form since arriving at Old Trafford in January 2018 and has been strongly linked with a move away before the European transfer window closes early next month.
The 30-year-old has managed just five goals in 45 appearances for United and has not featured since last season due to his Copa America commitments with Chile.
But Solskjaer shot down claims Sanchez has been told to train away from the first-team squad and said he expects the ex-Arsenal ace to feature heavily over the course of the season.
"Alexis is such a professional. He comes in working every single day, really hard," Solskjaer said at Friday's news conference.
"He wants to be part of this. These stories that he's been put in the reserves - of course he hasn't.
"He's part of our squad and he's a very good player. He's a few weeks behind the rest but he's very close to being part of it.
"We don't have the biggest forward line, so Alexis might end up playing a lot more games than you expect. We expect him to come good at this club. He's quality."
Brazil midfielder Fred is another player to have made minimal impact at United and did not even make the bench for last weekend's 4-0 win against Chelsea.
Phil Jones, Chris Smalling and Marcos Rojo were other high-profile casualties on the opening weekend, leaving Solskjaer to talk up the depth of his squad ahead of Monday's trip to Wolves.
"The good teams I've played in, we've always had a good keeper and a settled back line," he said.
"In Fred we believe he will have a big season as well.
"Every game is a puzzle because you have good players everywhere. Last time we couldn't fit Smalling, Fred, Marcos, Alexis and Jones into the squad of 18. It's a puzzle all over the pitch, but a nice one to have."