It's true African talents usually stand out in youth football due to their genetics so it's harder to evaluate them but that "phasing out" risk is not only exclusive to them. Just look at the list of flopped La Masia kids over the years. For every Busi or Pedro (so guys who were not hyped at all as teenagers) you have tens if not hundreds of likes of Bojan and G2S who were overhyped since they were 13-14, had a breakthrough to the first team but then their careers went downhill. And ofc many players didn't even make it that far. Including whole generation of Eto'os prodigies like Dongou, Bagnack, Ondoa,... And this whole overhyping of youngsters is much worse nowadays in social media era than it was 10-15 years ago.
Anyway, this signing is fully in the zero risk, potential high reward category. If we've found another gem then great if not we'll move on, maybe earn some money with him and find someone else. Odds that he'll make it here are probably low but you never know and these are the signings club has to make, especially in current financial situation. Also don't need to be said we've had lot more success lately with cheap signings than spending big money on players.
That said, Diarra looks like an inteligent player to me so it's not just his physics that stand out (which is more a case with some of his team mates). Don't think his longterm future is at winger though. I think he should be more involved in the game so moving him to midfield longterm makes sense. But my evaluation is based on only one game so could be wrong. I'm looking forward to see him in their next game(s).