Like most primarily offensive players, Coutinho runs hot and cool. Although he has been a very fine players for years, in his case this means that in lots of games he is a strong contender for Man of the Match, but in others he is just OK. He has streaks. He started this season cool, got red hot, and is currently cool again. This does not mean he is a bad fit. Unlike in the case of Cesc, I could tell right away that Coutinho is an excellent fit for this team. I expect him to be one of the best Barca players for many years.
This variability is not Valverde's fault, this is the way football works for most good players. Some are extremely good almost every game, but this is a very small list, the contender for the best player in the world list.
If Coutinho can start about every third game for a while, getting into all the other games as a sub, that's fine. He will carry this club again this season, just be patient.