So assuming we lose Chadli and Pritchard then we would have:
Eriksen, Alli, Lamela. Three first class attacking midfielders who we are lucky to have. After that there is:
Son - up and down first season. Clearly has some promise and is currently cover for Lamela
N'Jie - completely unproven. Is he an attacking midfielder, a centre forward or neither? I wouldn't yet be comfortable with him starting in a PL game.
Onomah - I don't see him as being an effective 7,10 or 11. I think he will be a number 8.
Mason - maybe number 10 would be a better role for him than the 8 role that he tended to play when picked last season?
My opinion here is that other than our first choice 3 and (perhaps) Son, I wouldn't really want to see any of the others starting games for us in the 7, 10, 11 roles if we are playing against anyone other than cannon fodder.
N'Jie may well come good and Mason could prove a revelation at number 10, but we haven't yet seen evidence that this will happen. Another specialist number 10 in the squad would be a wise move I think.