Kobbie Mainoo has been left out of Manchester United's matchday squad to face Aston Villa in the Premier League on Sunday. The midfielder was expected to at least be among the substitutes but was a surprise omission from the Red Devils' travelling party. A reason has now been given for his absence as United look to kick off the festive period with a win on the road.With Casemiro suspended and Bryan Mbeumo, Amad Diallo and Noussair Mazraoui all at the Africa Cup of Nations, United head coach Ruben Amorim was expected to shuffle his pack at Villa Park, with Mainoo tipped to come into the starting lineup. However, his name was not present on the team-sheet when the two starting lineups were revealed 75 minutes before kick-off.Mainoo had to withdraw from the United squad to take on Villa after picking up a small injury in a training session on Saturday. Much of the buildup to this fixture had centred on the 20-year-old after his brother wore a T-shirt which read 'Free Kobbie Mainoo' to Old Trafford for last Monday's 4-4 draw with Bournemouth.United legend Roy Keane slammed Mainoo's brother for these antics. "When he's got his idiot brother doing all that stuff - we shouldn't even be giving his brother the time of day. Because sometimes you're just surrounded by idiots, especially in families," Keane said on the Stick to Football podcast. "You come in after the match, and your brother did that, if my brother even did that, you would be looking at him going, 'what are you doing?' You would, and do you know what, do you think his brother's done that without asking him? He's under contract, probably being paid a decent wage."He's 20 years of age, what's wrong with sitting and learning your trade? And even if he's not getting a chance, we've all had to do it. Sometimes a manager is on your case and what you have to do is prove them wrong. Every day is your chance to prove yourself to the manager. He's got to get that in his mindset. The life of a footballer is about trying to prove people wrong. I have no problem with players going out on loan, it can work for everybody, but sometimes as a player, your biggest challenge is here at Man United and getting into that first-team. Even if the manager is saying 'you're not going to get in over Bruno,' prove them wrong!"Looking for smarter football bets? Get expert previews, data-driven predictions & winning insights with GOAL Tips on Telegram. Join our growing community now!Mainoo wanted to leave United on loan during the summer transfer window after seeing his playing time reduced under Amorim's management. The Carrington graduate ended up staying at the club, but has only played 302 minutes of football across all competitions so far this season and has once more been linked with an exit ahead of the January window.Amorim has often been quizzed on Mainoo's role and at times seemed critical of the midfielder, but he sympathised with the homegrown star over his brother's actions. At a press conference on Friday, the United boss said: "It was not Kobbie who wore the T-shirt. No [it won't affect team selection]. He is going to play if we feel he is the right guy. That is not an issue. I am not going to do something to Kobbie because of someone in his family. I'm used to it, I've been here for a year. He played really well, that is the important thing, my office is always open, nobody went there this week so everything is normal and we are ready to go to Villa."Add GOAL.com as a preferred source on Google to see more of our reportingWhile Mainoo has dropped out of the United squad, summer signing Benjamin Sesko will make his first start since their 2-2 draw at Nottingham Forest on November 1. Manuel Ugarte has been drafted into the XI in place of Casemiro, while a thin bench features two goalkeepers in Tom Heaton and Altay Bayindir, as well as four academy products in Tyler Fredricson, Jack Fletcher, Bendito Mantato and Shea Lacey.