Dembele is playing, and no, our squad is nowhere near wafer thin, quite the contrary in midfield actually. Granted, I would've liked to see more of Aleña, but I can see where EV is coming from, when he's playing first teamers ahead of him. Hopefully the rumors about EV wanting to slim down the squad for next season are true, which in turn should equal more chances for Aleña.
Our squad is most definitely wafer thin, compared to any reasonable standards. Depth of a squad isn't judged by how many players you have, but how many players of quality, and what the difference between the bench and the XI looks like. Not just compared to other big teams, even mid table teams (like Sevilla, who had big names on the bench against United) have a better squad than us.
Considering Busquets, Iniesta, Rakitic and Coutinho would be your starters, you're looking at mediocre to garbage players like Gomes, Vidal, Denis and Paulinho as your 'squad depth'. Then, take note that each of these guys have been frozen out by Valverde at various points of the season and not even made the squad lists.
If you can't manage to get youngsters playing time when you have such a poor squad, it's difficult to imagine that with the arrival of Greizman and maybe Arthur in the summer, Alena is going to see much playing time.
Lastly, Dembele is playing now, but it's possible that his playing time suffers too, and he's given stop start minutes once Greizman comes. Just look at Semedo at the start of the season.