Lukaku has praised Mourinho for his directness, and has referred to his conversations with Tottenham striker Harry Kane and his club and England international teammate Dele Alli in the Amazon series All or Nothing.
The 27-year-old Belgium international striker joined Italian giants Inter from Premier League powerhouse United in the summer of 2019 for a transfer fee reported by BBC Sport to be worth £74 million.
Lukaku told The Times: “Jose, if he sees something he doesn’t like, he will call you in his office. He either does like the conversation he did with Harry Kane or the conversation he did with Dele Alli.
“You’ll know where you stand. You have to be mentally very, very strong to be around Jose. I never really had a problem.”
Succeeding at Tottenham Hotspur
Mourinho took charge of Tottenham under difficult circumstances in the middle of last season, with the team struggling for positive results.
Finishing in the top four of the Premier League last season was not possible, especially with the injuries that Spurs had, but the former Chelsea and United manager did manage to guide the team to the group stage of the Europa League in 2019-20.
Tottenham are aiming to finish in the top four of the Premier League table.
Spurs will take on Manchester United away from home at Old Trafford in the Premier League on Sunday evening.