Unless you want Alenya to play as a winger, they don't.
Even so, Denis mostly plays on the left, not right (like Alenya).
Had a brainfart and thought Denis was left footed for some reason. If Valverde exclusively sees him as AML/ML backup for Coutinho then fair enough. We don't have anyone there.
But this end of LaLiga he has been mostly an option at MR and AMR replacing Dembele. If those are the positions that Valverde considers him at for next season then I much prefer it invested into Alenya.
I love Denis attitude, but also a lot of his work looks great with dribbles, but then ultimately ends in nothing as he will fall over, make a poor pass or shoot nowhere near on target. If a club like Napoli are willing to pay a good sum then it can be invested into a more impactful player. And I think he can easily flourish at a lesser side (look at what Gerry D managed at Milan).
Of course if we were to keep one of Denis, Vidal and Gomes then the answer is easily Denis.