Manchester United boss Erik ten Hag says Marcus Rashford's absence from their FA Cup fourth-round victory at Newport is an "internal matter".
The England forward missed training at United on Friday after reporting ill.
However, media reports then emerged the 26-year-old had been seen in nightspots in Northern Ireland on both Wednesday and Thursday evening.
"He reported ill. The rest is an internal matter and I will deal with that," said Ten Hag on Sunday.
When questioned why someone reporting ill should be an internal matter in his post-match news conference, United's Dutch boss simply replied: "It is an internal matter."
Asked whether that posed more questions than answers, Ten Hag added: "For you, not for me. It is an internal matter and I will deal with it. We will deal with it. I will deal with it."
Speaking on BBC One, former Newcastle and England forward Alan Shearer said: "If it is an internal matter you would think something is up.
"There is a huge talent in there with Rashford. We saw him be disciplined last season when he was late for a meeting and he actually missed a game but something is clearly wrong, either at home or with his relationship with the football club, because he can't keep doing this.
"He can't waste his talent, it's not right and he needs strong management and someone to get hold of him and say, 'you know what, if you get to the end of your career and have huge regrets, you can't have that'."
Rashford scored 30 goals in all competitions for United last term and signed a new contract with his boyhood club in the summer.
However, he has managed just four this term and his recent celebration, after scoring in the 2-2 draw against Tottenham, was criticised by former United captain Roy Keane and ex-Spurs midfielder Jamie Redknapp.
Also speaking on BBC One, former England defender Micah Richards added: "Marcus needs to understand he is a public figure and wherever he goes there is going to be attention on him.
"Whether he is given a day off, or whatever, but then before a game he is causing more problems.
"They wanted to come to the game today and focus on it and now we are having to ask him [Erik ten Hag] what is happening off the pitch."
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