Newcastle took the lead at Old Trafford in the reverse fixture before a rampant United side responded with four goals to claim all three points.
Mourinho bemoaned his side's defending for Ritchie's victor, and contrasted his own players' lack of urgency in that situation to the frenzied effort that secured Newcastle all three points.
"It was a case of waiting because we have 15 days now before the next game so we can keep working with him".
"Man United have some world-class players, but it seemed like we just wanted it a lot more". "We have played against Arsenal and Man City and you can really see the quality, and one mistake or a lack of concentration and a goal will go in".
"To me, growing up, Man United were the biggest team in the world and they probably still are", he said.
"We did it most of the time, and at the end we got that goal and we defended as a team".
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"It was going to be pretty tough at the beginning", said forward Perez. We will have some limitations and some problems, but you could still see this was another game when they were giving everything, and that is the positive thing.
But the cold reality is that United, with 27 games of the season gone, still don't have a known best starting XI, or even a formation that Mourinho and his staff have agreed upon.
"This is the first win since October against Palace, so that means a lot. By the way, thank you to those who travelled to Newcastle, you deserved a different game and a different score".
Manager Rafael Benitez added: "If you talk about players that are important for the team, normally people talk about goalscorers, midfielders and number 10s but in modern football, there are not too many consistent left-sided full-backs".
It gives Slimani the best part of a fortnight to work on building up his fitness for when Newcastle return to action against Bournemouth at the Vitality Stadium on February 24.
"He makes good runs and he's got a bit of everything to his game which will help us".