"We need the same spirit as on Sunday [against Spurs]".
Mourinho's position at the club has been under scrutiny this season with United now eighth in the Premier League, eight points off the top four.
They are looking to win the league for the first time since former boss Jurgen Klopp ledf them to the title in 2015/16.
The Red Devils are struggling to score goals, too, given the well-documented struggles of marquee signings Romelu Lukaku and Alexis Sanchez over the past 12 months.
Mendes said: "There have been more rumours of Jose Mourinho leaving Manchester United".
Manchester United made alarming defensive errors to allow Arsenal to take the lead twice either side of half-time, but goals from Anthony Martial and Jesse Lingard helped the home side to rescue a point. He said, "Yeah, definitely". In fact we may even start keeping a few more clean sheets with Koscielny leading from the back....
"At the moment we are talking about it being a feat if they get into the top four so this is a must-win game. That's the main thing".
"I don't know, the training session [on Friday] will give us some answers", said Mourinho.
He said football has "changed" and that United have "no chance in the market unless you go to insane numbers".
The Red Devils are now eighth in the Premier League table and are without a win in their last four league games.
Eric Bailly, who impressed in the draw against Arsenal earlier, was also not seen arriving with United's squad and is now considered a major doubt to feature against Fulham.