Lukaku claimed the Special One would have done far better at Old Trafford if he'd been given the resources he needed.
"Jose Mourinho - if he would have had the players that he wanted, he would have done better than what we did."
As well as his Mourinho thoughts, Lukaku also revealed he told United he wanted to leave as far back as March.
He added: "I had to re-find myself.
"Last year was a difficult year for me on the professional side because I think it was the year when stuff wasn't going the way I wanted and I was not performing as well.
"So I had to find in myself what was lacking so I came to conclusion it was time to change environment.
"I made my decision around March and I went to manager's office and told him it was time for me to find something else because I wasn't performing and I wasn't playing.
"A lot happened in that year and it was better for both sides to go separate ways.
"I think I made the right decision and Man Utd has made space for the younger players to come through.
"So I think it was a win-win situation for both of us."