Erik ten Hag insisted he will deal with Marcus Rashford's absence after the Manchester United star missed Sunday's 4-2 victory at Newport in the FA Cup.
Ten Hag claimed Rashford was absent from the fourth round clash in south Wales after saying he was ill on Friday.
But reports over the weekend suggested the England forward had spent Thursday evening at a Belfast nightclub before missing training with United the next day.
"He reported ill," United boss Ten Hag said after United blew a two-goal lead before surviving their trip to fourth tier Newport.
"The rest is an internal matter. I deal with it, we will deal with it."
Ten Hag said there was a "no good culture" when he arrived at United and he has encountered disciplinary issues during his Old Trafford tenure.
United winger Jadon Sancho returned to Borussia Dortmund on loan earlier this month after falling out with the Dutchman.
Rashford was also dropped for a Premier League match against Wolves last season after turning up late to a team meeting when he overslept.
Pressed on his latest issue with Rashford, Ten Hag said: "I don't go in this case. We talked before about it, we played a good game, and now we move on."
United struck twice in the first half against opponents 76 places below them in the pyramid thanks to goals from Bruno Fernandes and Kobbie Mainoo.
But Bryn Morris and Will Evans scored either side of half-time to leave United in danger of a humiliating defeat.
Antony and Rasmus Hojlund struck in the final stages to spare United's blushes, but it was another disjointed display in a difficult season for Ten Hag.
"The first 35 minutes it was very dominant. We did not give the opponent any chance and should have been three, four, five-nil up," Ten Hag said.
"Out of nothing they score a goal and straight after half-time again.
"We have to be critical of bad defending in transition. Poor defending from the cross, it's 2-2, and they turned it around.
"But you see how resilient we are. We stayed calm, went back in our game, got the third and fourth goal, so job done."
Casemiro, Lisandro Martinez and Luke Shaw were all back in the United starting line-up after injury, while Harry Maguire made a late cameo for his first appearance since December 12.
Ten Hag hopes the return of those experienced stars can help a team who are eighth in the Premier League and have been eliminated from the Champions League.
"I know the character from Licha (Martinez), Casemiro, Luke Shaw, (Raphael) Varane, Bruno, Antony, (Alejandro) Garnacho, Hojlund. All fighters," Ten Hag said.
