During the first half of the campaign, Mourinho acknowledged that the 22-year-old could arrive back at Old Trafford in January if United felt that his return would benefit both parties.
However, with Pereira continuing to gain regular action at the Mestalla, Mourinho has opted to allow the Brazilian to stay in La Liga.
The 54-year-old is quoted by The Mirror as saying: "They're playing in a total different system than us and he plays in a position that we don't have. We will need an update in relation to that.
"But the experience he's having, playing in a good competition, in a good club, in a difficult club because it's a club with big fanbase, with pressure around, I think is an amazing experience for him and we are happy to let him [stay] there."
Pereira has scored one goal in 19 appearances in all competitions this season.