It's a shame but I don't think he will make it here.
Do we cash in this summer, or give him another season and see how he develops but we may lose him for nothing.
I would take the risk of using him far more often next season. My thinking is basically the footballer's age curve. Too often we make judgements based on how a player is performing right now, rather than how you might reasonably expect them to do in the next two or three years. Someone aged 30 will probably decline in that time frame, whereas a player who has La Liga and PL experience aged 23, will probably be a decent player in their mid 20s, assuming they have the right attitude and avoid serious injuries.
Andreas has tons of technical ability. He has demonstrated a willingness to work hard for the team, in recent matches against Liverpool and PSG. He can produce moments of magic, such as his goal against Southampton. What he still lacks is midfield wisdom. Knowing the appropriate choice for different situations on the pitch. That should come with age and playing time. He played wide left most of the time for Granada and wide right/left for Valencia.
I admit that his performances this season have not been amazing. And he may be frustrated at his lack of game time.
As things stand, I would have McTominay as a regular starter, Matic and Fred sharing one position, with Andreas as back-up for Scott and Paul. I could see Pereira making 15 to 20 PL starts under that scenario.
If we buy another central midfielder to replace Ander Herrera, that leaves Pereira in a difficult position, with limited opportunity to play. Should Solskjaer want to keep Andreas for next season, he may need to convince Pereira that he will get far more opportunities to play.