As said before I see him as a Vidal replacement. Someone who can provide goals but also has good work ethic. I think he's acutally underrated defensively.
I'll let Koeman (and whoever our coach will be in future) worry how to fit players fit into the lineup but we need more than just 3 starting quality midfielder and Donny has enough quality to start. And we also need different profiles of midfielders.
FDJ, Busi (Ideally we'd also replace him with Camavinga but won't happen), Pjanić, Donny, Puig, Pedri looks good for next season, now it's up to Koeman to find the right combination of players for different opponents. We'll play a congested fixture and there will still be 5 allowed subs in games so enough chances for everyone.