According to Neil Custis and Richard Moriarty at The Sun, Sanchez sent a WhatsApp message to team-mates telling Marcos Rojo he owed the Chilean £20,000 after Mourinho departed Old Trafford on Tuesday.
Sanchez was brought to Manchester United from Arsenal by Mourinho in January 2018 in a swap deal involving Henrikh Mkhitaryan.
The deal saw the Chilean become the Premier League's highest earner. He is on wages of £350,000 per week with the Red Devils, according to Goal.
However, he has struggled for form since joining the club. He managed only three goals last season for United and has scored only once in the current campaign.
Football writer Richard Jolly offered some context on his goalscoring record in 2018:
Sanchez told friends he wanted to leave Manchester United for Paris Saint-Germain after being dropped for the club's UEFA Champions League game against Young Boys, according to John Cross at the Mirror.
The forward was then ruled out for the rest of the year after suffering a hamstring injury in training in November.
The 30-year-old is currently back in Chile recovering from the injury. The club hope the break will also help "revitalise" his Manchester United career, according to Miguel Delaney at The Independent.
Former Arsenal midfielder Emmanuel Petit has offered his view on Sanchez's situation:
Chris Wheatley @ChrisWheatley_
Emmanuel Petit on Alexis Sanchez: “He wanted to leave Arsenal because he was chasing the salary United could offer. Now he receives so much money and in nine months what has he done? Nothing. I have not seen him. I think they signed his brother.” https://t.co/Zke1h0TGrz
There's no doubt that Sanchez has been a flop at Manchester United so far. Yet the departure of Mourinho offers him another opportunity to make a mark at Old Trafford.
Caretaker manager Ole Gunnar Solskjaer told MUTV (h/t the Guardian's Mark Dobson), his players will start with a "clean slate" and he wants the team to "enjoy" their football again.
Manchester United will be hoping Solskjaer can bring attacking, entertaining football back to Old Trafford. Sanchez could still have a key role to play with the Red Devils if the new manager can find a way to bring out the best in the attacker.