Arsene Wenger says Alex Iwobi will be fined after partying until the early hours of Saturday morning ahead of Sunday's humiliating FA Cup defeat to second-tier Nottingham Forest.
Iwobi, 21, was filmed dancing in central London at 2.37am on Saturday in a video uploaded to Snapchat, according to The Sun newspaper.
The Nigeria international played 90 minutes of the 4-2 loss to Forest at the City Ground, the first time Wenger had experienced exiting the FA Cup - a competition in which Arsenal are the holders - at the first hurdle.
When asked if Iwobi faced disciplinary action, Wenger said: "That is true. He will be fined. It is impossible to go out 48 hours before a game. It is unacceptable."
In a wide ranging news conference on Tuesday held at the club's London Colney complex, the Arsenal manager also discussed the futures of several first-team players including Alexis Sanchez, with speculation growing that Manchester City are set to revive their summer interest in the Chilean forward.
Wenger said he is yet to decide whether Sanchez will start Wednesday night's League Cup first-leg semi-final against Chelsea amid speculation the Arsenal forward could join City this month.
Iwobi was reported to have attended a party that included people smoking cannabis and inhaling laughing gas.
But Wenger said his player was not involved in either taking drugs or drinking alcohol.
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"First of all we have to be cautious with everything that is coming out," Wenger said.
"When you read what is reported it is like he has taken drugs. He was not involved in that at all.
"He has been out at a birthday party and how long he has stayed? That is important. After, I will decide what happens.
"Did he do drugs or alcohol? Not at all. If your neighbour takes drugs, what can you do? You are responsible for your own behaviour, not the behaviour of your neighbour."
Per Mertesacker had said after the Forest defeat, in which Wenger made wholesale changes to the starting XI, that no player deserved their place in the team. Wenger admitted Iwobi's performance against Forest may have been affected by his actions in the early hours of Saturday.
"Who knows that?" Wenger said. "You know that sometimes you want to prepare 100 per cent but it is not always a guarantee you will be at your best.
"But that is what is requested from a top-level professional."
Wenger said after the match that the club would look to add to their summer spending with Greek defender Konstantinos Mavropanos the only new signing so far, but on Tuesday he was forced to field questions about his own players.
Sanchez was the subject of repeated interest from Premier League leaders City over the summer, and with the former Barcelona player entering into the final months of his contract the club may be forced to accept a cut-price deal or risk losing Sanchez for nothing in the summer.
Wenger insisted no club has yet made contact over Sanchez but, asked if he expects the Chilean to stay, Wenger said: "I expect nothing."
Another player who could leave is Francis Coquelin. Wenger said "it is a possibility" the midfielder could be let go but said reports suggesting a deal is agreed with West Ham United are wide of the mark.
Arsenal will be looking to bounce back from the Forest defeat at Stamford Bridge but they are not expecting any injured players back for the first leg.
Granit Xhaka and Shkodran Mustafi will both undergo late fitness tests.
"We take the criticism on board and respond to it in a positive way - that's all you can do," Wenger said.
"We're in the semi-final of the League Cup and we have an opportunity to go through."