Ole Gunnar Solskjaer has reportedly pulled Jesse Lingard and Marcus Rashford aside to warn them to focus on football and not fashion.
Midfielder Lingard owns merchandise brand JLINGZ - which sells various pieces of clothing, including hoodies, t-shirts and shorts, alongside various accessories like phone cases and caps.
Last Christmas, the 26-year-old launched his own label called "Be Yourself".
There is also a worry in the United camp that striker Rashford is losing focus on football, which could be exacerbated after he suffered a groin injury during United's 2-0 defeat to West Ham .
According to the Sun, Solskjaer and No.2 Mike Phelan have both held talks with the pair as they attempt to improving the dressing room spirit at Old Trafford.
Lingard was criticised last year for his fashion launch, which came in the week before a 3-1 defeat to Liverpool - when Jose Mourinho was still in charge.
United legend Roy Keane ranted at the time: “If there was a good strong dressing room that wouldn’t be tolerated. That’s why I worry about the United dressing room.
“That wouldn’t be tolerated in a good dressing room from a young player, who’s still learning his trade.
“Again, he could be the nicest kid in the world, I don’t know the lad, but if you’re coming out with all that nonsense...”
Lingard has appeared in six Red Devils fixtures so far this season but hasn't found the net since January, with his last Premier League strike coming against Cardiff in December 2018.
England star Rashford has netted three times this season, but only two of those came from the penalty spot.
And former boss Mourinho explained on Sky Sports how to get the best out of Rashford.
Mourinho said: "The most important match of my period was the Europa League final. He [Rashford] was my No9.
"He can be a No9, he can be dangerous.
"But when United plays against teams that go to Old Trafford and 'park the bus' he is not a striker to play with his back to goal.
"From the side, he would get 10-12 goals per season. He works for the team from a defensive point of view.
"Playing from the left, from a Mane position, he can be a better player than as a No9.
"He feels he is a No9 and the club and manager feel that."