It’s not the first time that family members have involved themselves in matters at United, with Alejandro Garnacho’s brother playing a central role in the saga which led to his messy summer exit.

Towards the end of the winger’s last season at Old Trafford, his brother Roberto waded in to criticise Amorim on social media, accusing the Red Devils manager of throwing Garnacho under the bus after the Europa League final defeat to Tottenham.

He wrote: “Working as no one else, helping every round, coming from two goals last two finals… just to be on the pitch for 19 minutes and get thrown under the bus.”

The situation which led to Garnacho’s exit was nearly identical to the one involving Mainoo. Like his former team-mate. he is clearly frustrated over his playing time and the situation risks turning ugly if his brother keeps fighting his battles for him.

It seems to be a successful tactic, given that Garnacho eventually got his own way by securing a move to Chelsea. Mainoo may be hoping that a similar approach could help to engineer a January transfer, with Napoli thought to be among the interested parties.

The midfielder was denied a loan move away from Old Trafford in the summer but is still believed to be keen on leaving United next month. Maybe he feels that kicking up a fuss would be the best way to force Amorim’s hand and get what he wants.

It is not known if Mainoo was involved in his brother’s decision to wear the shirt, or if he decided to wear it on his own accord. Either way, it’s not a good look for United.

Succumbing to player power makes them look weak and tells other members of the squad that throwing their toys out of the pram is a successful way to undermine authority and get their own way.

The situation risks becoming a farce and Amorim needs to take control before it spirals into another needless drama, which United could really do without as they look to return to the Premier League’s upper reaches.

Amorim has already invited Mainoo to speak with him about his frustrations, saying before Monday’s game: “If Kobbie comes to me and talks to me, I will talk to him.

“I’m not going to say what I’m going to say to Kobbie but I will be really pleased if Kobbie comes to talk to me about that. I just want my players happy and I understand that every individual has their goals.”