He's worked his way out of the u21 team and has been training and playing with the first team for a while now, he's proven himself to Glasner, not saying it's impossible but he'd have to do a lot in training to show he's on the same page as Glasner and how the team play.
no disrespect to devenny at all, even got him in my fpl team cos i assumed eze'd leave. he's still pretty raw while mainoo has more experience and has different attributes that fit your christmas tree formation. having said that i dunno if palace just want to buy players from europe for lower wages.