The Red Devils have struggled at the start of the season and already sit 12 points behind Premier League leaders Manchester City.
It comes in Jose Mourinho’s third season in charge at Old Trafford, with the former Chelsea manager yet to guide United to the Premier League title.
Mourinho has reportedly fallen out with a number of players at Manchester United.
It includes Paul Pogba and Alexis Sanchez, while he has criticised the likes of Luke Shaw and Anthony Martial.
And Lukaku has slammed his team-mates, believing they should stand up for themselves and ‘deal with it’.
“I think he has a good relationship with the players,” Lukaku told Bleacher Report
“I don’t really pay attention to what his relationship is with the other players, to be honest.
“I don’t think it’s something that should affect me.
“If he [Mourinho] has an argument with somebody I mean, deal with it, man.
“You’re a grown-a** man, deal with it.”
While Mourinho is yet to win the Premier League with Manchester United, he has helped the club to the FA Cup, League Cup and Europa League.
But former Chelsea striker Didier Drogba believes he would have won the title if he was at Manchester City instead.
“I think if you put [Mourinho] at Manchester City, he would have won the league twice, maybe two or three times,” Drogba told the Mirror.
“You get criticism because you set a standard of winning, winning and winning and now you’re not winning or having the same results.
“He is still [at Manchester United]. You have to look at all the managers who were at Manchester United the last few years.
“Being manager after Sir Alex Ferguson is not easy and the finances that they had then are not the same as what they have now.
“People only pay attention to the best and he’s one of the best and that’s why he gets all of this.”