Fellaini told John Cross of the Mirror that the 56-year-old was not given enough time to build a new team at Old Trafford.
"Mourinho, for the first season, he did amazingly, he improved the team, won things. OK, the second season, was a bit more difficult but he tried, you know, and did his best to help the team. Then they decided to sack him and that's football.
"For me, yes [they were too quick to sack him] because a manager like that comes and he needs a lot of players to do his own philosophy. He wanted to build his team and after two and a half years they decided to sack him because the results were not there.
"They brought in one of the best managers in the world in Jose Mourinho, he wanted to build a team and they sacked him. It’s not easy to build a team in that time, you need more than two years.
Fellaini, who left Manchester United for Chinese Super League side Shandong Luneng in February, was also critical of some of the players at his former club, per Matt Law at the Daily Telegraph.
“You see some players go straight into the dressing room after games and go straight onto social media," he said. "It’s wrong."
Mourinho won the League Cup and UEFA Europa League in his first season at Manchester United and finished second in the Premier League the following year behind Manchester City.
However, he was sacked in December 2018, his third season, after a 3-1 defeat to Liverpool saw the Red Devils slip 19 points off the top of the table.
Opta pointed out it was the club's worst start to a Premier League campaign since 1990-91:
Mourinho's Manchester United reign was also notable for clashes with key midfielder Paul Pogba. The Portuguese frequently dropped the World Cup winner and clashed with him in training, as shown by Sky Sports News:
Mourinho was replaced on a caretaker basis by Ole Gunnar Solskjaer. The former striker initially oversaw an upturn in results but has seen his team's form deteriorate since being made permanent manager in March 2019:
Manchester United were beaten 1-0 by Newcastle United on Sunday and head into the international break in 12th place in the table and just two points off the relegation zone.
Opta noted it is the club's worst start since 1989-90:
Manchester United's results and poor performances mean that Solskjaer is already coming under pressure after only seven months in the job. Former Juventus boss Massimiliano Allegri has been linked with replacing him in the Old Trafford hot seat, per La Gazzetta dello Sport (h/t AS).
Yet reports have also suggested the Red Devils are willing to back Solskjaer in the transfer market with four signings in the next two transfer windows, according to Jason Burt of the Daily Telegraph.
Solskjaer will need results to improve if he is to see out his three-year deal, and he faces another difficult test next time out, with unbeaten Premier League leaders Liverpool visiting Old Trafford.