Fellaini struggled to make a positive impression early in his United career after arriving from Everton for £27.5 million (31.2m euros, $36.8m) in 2013, with one British newspaper placing him among the 10 worst buys of the Premier League season at the end of his first year.
Yet United manager Mourinho has made clear he sees Fellaini’s contribution as vital to the club’s progress, saying last month he felt his team was “weaker” without the midfielder in it.
The Belgium international missed Wednesday’s 4-1 Champions League win away to CSKA Moscow with an ankle problem, but returned to score twice as United beat Crystal Palace 4-0 at Old Trafford in the Premier League.
“I think only a strong character could resist the difficulties here,” said Mourinho.
“He had some difficult times when people didn’t recognise his qualities, when other managers didn’t like so much the qualities he has.
“He is a fighter, a guy with lots of pride and I am really pleased I helped him reach this level and change the perception the fans have now.”
Mourinho suggested Fellaini was benefiting from his management more than he had done under his predecessors at Old Trafford.
“All managers are different. Sometimes we like players that others don’t like,” said Mourinho.
“Some players perform better with some of us than with others –- and I have examples of players that didn’t perform well for me and then performed well for other managers.
– ‘We took blows to the chin’ –
“I think Marouane has important qualities and I try to use them depending on the situation and what the team needs.
“He plays with me as a defensive midfielder, as a second striker, as one of two midfielders. He is a great player. A great character.”
United are in a strong position in the Premier League going into the international break, with four wins out of four at home, but Mourinho is not getting carried away.
“It is just four matches at home,” said Mourinho.
“I know that in the same four matches at home last season we got eight points, not 12. We have four more points at home than last season.
“We are playing well. I don’t say that we played well for 90 minutes because sometimes we have periods where we lose a bit our consistency, our intensity. But it is difficult to do that for 90 minutes.”
Palace remain bottom of the Premier League, with no goals and no points after seven matches, with this defeat coming seven days after they were beaten 5-0 at Manchester City.
Manager Roy Hodgson admitted he faces a tough fight to avoid relegation, but believes it can be done.
“We have to accept we are a boxer fighting in a weight class we are not able to handle at this minute and we took blows to the chin and got knocked down,” said the former England manager.
“But the lessons we are learning could stand us in very good stead. We are not going to have 38 games against Manchester City and Manchester United.
“If we can stop the players doubting themselves too much, who knows, we may look back on this terrible spell and say it helped us build the character to stay in the league.”