The 21-year-old Brazilian defied Mourinho’s express advice when he sought, and accepted, a temporary move to the La Liga side, having spent all of last season on loan at Granada.
Pereira, signed as a youth player from PSV Eindhoven five years ago, has yet to start a league game for United but is so highly regarded that he was handed a new, long-term deal by Mourinho this summer, just as he decided to move elsewhere.
“His was a personal decision that I don’t agree with, a decision I don’t think honestly is the best decision for him, a decision that disappoints me,” Mourinho said at the team’s Carrington training base on Friday.
“He has the potential to be fighting for a position, fighting for opportunities and fighting to be a Manchester United player.”
Given the depth of the squad Mourinho has at his disposal this season, it is not hard to understand why Pereira might have thought he would be better served by moving elsewhere to gain more experience.
But Mourinho, who takes his table-topping side to Stoke on Saturday, added: “His decision can be considered a young player who wants to play every weekend but also a young player that is not ready to fight for something difficult.”
Mourinho also welcomed the decision made by Premier League clubs to bring forward the summer transfer window in 2018 to the Thursday before the new campaign kicks off.
But he is wary of the danger that some English clubs may lose players to foreign teams who can still sign stars until the end of August.
He said: “As the head coach of Manchester United, obviously I am in favour of that because if I focus just on the training process of course.
“I want to arrive, day one, I want to have my squad, my players, I want to work and travel with them in pre-season, I don’t want to be waiting for late decisions.
“But there is another perspective and I have to share the problems the board has and that is that obviously when the decision is not uniform around Europe and all the championships then we are opening a disadvantage for the English clubs.
“That was the reason why Manchester United voted against because there is another element – the market will be closed for us and is open for others and they can come to the English clubs.”
The Manchester United boss said he wanted a broader solution, adding: “I hope it goes to the perfect situation, which is to close for every championship and not just the Premier League.”
Philippe Coutinho has been left out of Liverpool’s squad to face Manchester City in the biggest Premier League match of the weekend, manager Jurgen Klopp said on Friday.
Klopp said the Brazil international, who was linked with a move to Barcelona following the departure of his compatriot Neymar to Paris Saint-Germain, was in “a good mood” after returning to Merseyside following international duty.
Coutinho has not yet played for Liverpool this season, with the club citing a back injury, but he joined up with his national side for World Cup qualifiers and made two substitute appearances against Ecuador and Colombia, scoring against the former.
But Liverpool’s German boss said the playmaker would not feature against City.
“We have got all the numbers (on Coutinho) from the Brazilian FA — the training, the intensity of the training, stuff like that, and it was all OK. And he played twice — I think about 15 or 20 minutes,” said Klopp.
“He could have played 15, 20 minutes again in the City game, maybe longer, but I think really it makes sense (to leave him out).
“He agreed completely, so it was not as if he said ‘no, I don’t need training, I want to play’. It was a good conversation.”
Liverpool, who thumped Arsenal 4-0 in their last match before the international break, are second in the Premier League after three matches, with two wins and a draw.
Pep Guardiola’s big-spending City have the same number of points but trail Liverpool on goal difference.
Top-four rivals Manchester City and Liverpool go head to head at the Etihad Stadium on Saturday.
Press Association Sport looks back at some of the best previous meetings.
MANCHESTER CITY 1 LIVERPOOL 4 (Premier League, November 21, 2015)
A result which highlighted just how good Liverpool could be when they got Jurgen Klopp’s tactics right.
City, title favourites at that point, were blown away in the first half-hour as Eliaquim Mangala’s own goal was followed by strikes from Brazilians Philippe Coutinho and Roberto Firmino.
The Reds did not crumble when Sergio Aguero pulled one back just before the break with Martin Skrtel smashing home the fourth, but it was more the manner of the victory rather than the scoreline which was impressive.
LIVERPOOL 3 MANCHESTER CITY 2 (Premier League, April 13, 2014)
Liverpool, under Brendan Rodgers, seemingly struck a decisive blow in the title race as this victory put them in pole position against their visitors.
Raheem Sterling and Skrtel put them 2-0 up only for David Silva and Glen Johnson’s own goal to level things after the break.
However, the drama was not over as Coutinho fired home the winner.
MANCHESTER CITY 3 LIVERPOOL 0 (Premier League, August 23, 2010)
City put on a show for owner Sheikh Mansour, the man who spent £1billion to get them where they are, who was present at a league match for the first time to witness their biggest win over Liverpool since 1937.
Gareth Barry gave the hosts an early lead before Carlos Tevez scored twice after half-time.
MANCHESTER CITY 2 LIVERPOOL 2 (Premiership, May 5, 1996)
Desperately fighting to avoid relegation on the final day of the season, City fans feared the worst when a Steve Lomas own goal and Ian Rush strike put them 2-0 down at half-time.
Uwe Rosler and Kit Symons levelled things up with 12 minutes to go.
Manager Alan Ball knew his side needed to do better than Southampton and Coventry to survive so when he got word Coventry were losing he instructed City to play out time.
Lomas was playing keep-ball near the corner flag when the recently-substituted Niall Quinn ran out of the tunnel to tell his team-mates they had been misinformed and needed to win.
Unfortunately for City they ran out of time.
MANCHESTER CITY 2 LIVERPOOL 0 (FA Cup fourth-round replay, February 7, 1973)
City were enduring one of those seasons which would become all-too familiar to their fans as they were knocked out of Europe and out of the League Cup (by fourth division Bury) by early October.
They were not given much chance against the favourites, who went on to win the league and UEFA Cup that campaign, but a goalless draw at Anfield took the tie back to Maine Road.
Colin Bell and Tommy Booth scored the goals which took them through at home.