Kobbie Mainoo's brother was pictured wearing a T-shirt which read "Free Kobbie Mainoo" as Manchester United drew 4-4 with Bournemouth in a chaotic game at Old Trafford on Monday.
Mainoo has struggled for game-time under head coach Ruben Amorim this season and has been linked with a move away from the club in January.
The England midfielder was introduced off the bench to loud cheers from the home fans in the second half against Bournemouth.
Amorim watched his team lead three times before having to settle for a point.
Only two late saves from Senne Lammens prevented Bournemouth from finding a late winner.
"It was a fun game for everyone at home," said Amorim.
"It was a fun game to watch, of course there is a feeling of scoring four goals and not winning the game.
"We are lacking quality when we defend our goal. It's something that we need to address of course."
Mainoo is yet to start a Premier League game this season, but came on for Casemiro in the 61st minute against Bournemouth.
During the game his brother, former Love Island contestant Jordan Mainoo-Hames, was pictured wearing a t-shirt printed with the message "Free Kobbie Mainoo."
The 20-year-old, who is keen to find more regular games in an attempt to force his way into England's World Cup squad, has been linked with a move to Napoli.
Sources have told ESPN that United are reluctant to sanction a permanent move, but could listen to loan offers in January.
It's likely that they will only consider letting him leave at the end of the window once Africa Cup of Nations players Bryan Mbeumo and Amad Diallo are back.
Amorim wasn't asked about the incident after the game, but the Portuguese coach will have the chance to address it at his weekly news conference on Friday.
Before the game, Amorim said: "First of all, I say the same thing: the team comes first.
"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 is coming to talk to me about that.
"I just want my players happy, and I understand that every individual has their goals. The frustration doesn't help anybody but, again, the focus is in this game and we will see if that happens."