Comparing Dembele to Ansu is no necessity, but you do have to consider the overlapping positions and general pecking order of our offense.
Good coaching starts with healthy experimentation. You can't know what works until you've tried different things to give you an idea. Our lineups this season so far do indicate experimentation, but what Valverde sorely lacks is the ability to recognize mistakes quickly and correct them immediately. FDJ's position should already be clear, for example.
Good coaching also means affording continuity to things that work, even if it was a bold idea in the first place. If Ansu extrapolates that 15 minute performance to regular substitute appearances then the right decision is to keep playing him and don't slow down. That's not even considering that he's 16 and rapidly growing in experience and maturity.
If Ansu can provide things that Dembele cannot then you push him up the pecking order, damning the status quo. That's not just in terms of better quality performances and better effort, both of which are highly possible that Ansu could do right now if Dembele doesn't stop spiraling. It's also in terms of profile, as Ansu is a more natural LW and even more importantly he looks super comfortable drifting ACROSS the defensive line wherever/whenever he needs to so he can also act as a goalscoring forward.
Dembele is injured right now, and Ansu just killed it in his first performance. Keep giving him minutes until it stops, ignoring the fact that he's only 16. If he proves that his value is competitive to Dembele even at his current inexperienced level, then we can't hesitate to give Ansu priority. Especially given his insane youth/potential, ability to maybe lead our attack one day, and background as a youth product from a young age who's fulfilling his dream. It's an added long term investment as well as short term, to frontload his progression.