Mourinho, 55, became increasingly spiky in his last few months at Old Trafford, lashing out at the board’s transfer policy and turning his fire on his squad, especially record signing Paul Pogba.
His constant complaints about the players’ lack of desire had an impact on the pitch, culminating in the 3-1 defeat by Premier League leaders Liverpool on Sunday which left United 19 points behind their opponents.
Mourinho’s sacking comes just two-and-a-half years into his tenure and three years and a day since Chelsea dismissed him just months after he had guided them to the title.
Former United striker Ole Gunnar Solskjaer, currently in charge of Norwegian club Molde, is reportedly being considered for the role of caretaker manager until the end of the season.
Mourinho’s dismissal saw shares in the US-owned club rise on Wall Street by more than 5% to $18.18.
Former players such as Rio Ferdinand and Patrice Evra expressed hope a change in management would bring a marked change in atmosphere.
“The club needs an injection of positivity and hopefully this decision will allow that to happen,” Ferdinand posted on Instagram.
Mourinho’s old sparring partner Pep Guardiola insisted he took no satisfaction from seeing his rival dismissed.
Guardiola and Mourinho were often at loggerheads during their time at Barcelona and Real Madrid respectively, but had a more cordial relationship in Manchester.
“When it happens I’m sad for the managers, all the time,” Guardiola said after City’s League Cup quarter-final win over Leicester.
“We are alone. For the manager, when the situation is not good, we are alone.
“You know what happens when you are sacked, because the results maybe are not good.
“Of course he doesn’t need luck because he’s so strong and I wish him all the best and soon he is going to come back.”
United’s worst start in the league since 1990 means even Mourinho’s ambition of finishing in the top four and a Champions League qualification spot is in danger.
The club has reached the Champions League knockout stages, where they face an intimidating last-16 tie with Paris Saint-Germain, but a combination of poor results, dressing-room discord and criticism of the board proved potent factors in the decision to dismiss the Portuguese manager.
A United source told AFP that Mourinho had been supported in his efforts to sign players and described reports of player power at the club as being key to the sacking as wide of the mark.
In the longer term former Real Madrid coach Zinedine Zidane has been linked to the club as has his fellow 1998 World Cup-winning team-mate Laurent Blanc, who played for United. Tottenham Hotspur manager Mauricio Pochettino is also strongly tipped.
One bookmaker makes Pochettino the early favourite although the 46-year-old Argentinian refused to be drawn on the speculation.
“We know a lot of rumours happen. But it’s not my business what happens in another club now,” he said at a press conference on Tuesday.
Mourinho’s reign had started well enough with the League Cup and the Europa League trophies but for a club that has been champions of England 20 times, neighbours Manchester City’s dominance over them in the league has hurt.
Despite his protestations to the contrary, the United board gave Mourinho plenty of money to compete.
Top-quality players such as Belgian striker Romelu Lukaku – who he persuaded to snub Chelsea and come to United for £75 million (B3.1 billion) in 2017 – and Pogba for a then world record fee of £89mn (B3.68bn) in 2016 have floundered under his authoritarian stewardship.
Indeed it was Mourinho’s deteriorating relationship with World Cup-winning midfielder Pogba – one of many to feel the lash of his tongue in public – that symbolised the decline at the club under him.
Mourinho questioned his attitude while the player hit back with open criticism of the tactics, culminating in him kicking his heels on the bench during the Liverpool debacle on Sunday.
The manager’s overall Premier League points haul of 176 since his appointment by United lagged behind City, Liverpool, Tottenham and Chelsea during the same period.
There was sympathy too for Mourinho, including from Jurgen Klopp, whose Liverpool side had sealed his fate and whose positive football exposed painfully the negative tactics of the United manager.
“Nobody can take away all the things he has won. I hope he has that on his mind as he leaves. He is an outstanding manager.”