midfielder is on the brink of a suspension after already accumulating four yellow cards this season. The 19-year-old's booking in Sunday's 0-0 draw against moved him one away from becoming unavailable ahead of a crunch run of fixtures.
United host Brentford, Chelsea and Leicester City at in the Premier League between the October and November international breaks.
But could be forced to go without his star midfielder for at least one of them after his yellow card at Villa Park.
Players face a one-match ban if they receive five yellows across the first 19 matchweeks of the Premier League campaign.
Chelsea Wesley Fofana and Marc Cucurella for the daunting trip to Anfield to face leaders Liverpool later this month.
Both players saw yellow for a fifth time this term during an ill-tempered brawl in Sunday's 1-1 draw with Nottingham Forest.
And United could soon also pay the price after a couple of reckless displays against Villa and Tottenham.
Rob Jones booked the Red Devils' players five times on Sunday as they attempted to keep the lively hosts at bay.
In the 3-0 defeat to Spurs at Old Trafford, six stars entered Chris Kavanagh's book on a dismal day for Ten Hag.
That included Bruno Fernandes, who saw red for a challenge on James Maddison. However, the decision was later overturned by the FA due to wrongful dismissal.
Another booking for Mainoo against Brentford on October 19 would be an almighty blow, ruling him out of Chelsea's visit to M16.
Harry Maguire, Lisandro Martinez (both three), Christian Eriksen and Mason Mount (both two) are United's other players on multiple Premier League bookings.
Marcus Rashford was fortunate to escape a second yellow card against Villa for a careless foul shortly after entering the book.
After full-time, Ten Hag admitted that his sudden substitution was with that in mind as another United player displayed their frustration.
"It was maybe a second foul, but that would be very harsh," the Dutchman explained.
"But, yeah, you can have the feeling in an away stadium and when you make another one [foul], then you have to hope for the refereeing, what they are doing.
"So I thought it was the best idea to take him off to protect the team but also to protect him."
