He's a central player and suited towards playing in the centre. He doesn't have a lot of pace so he's not suited to RW at a Mourinho team and at Atlético the wide players generally play like wide midfielders, utilising the fullbacks for width as other than Carrasco and Griezmann(who is at ST most of the time) they don't have much pace in wide areas.
Saul can also play at DM, CM and CB. I would say that Saul would be utilised in a similar manner to how Koke would be utilised, as a mixture of a creative player and a play breaker(so a no.8). You arguably have that already in Schneiderlin but I guess Saul has more potential in that regard.
The whole getting a solely creative midfielder thing that DirtyBit is going on about makes sense if Rooney wasn't in the side. However Rooney is in the side, you are probably going to get Ibra so there's no space for a creative midfielder in mourinho's system. A creative winger however there is space for, but you best be made sure that the winger will be tracking back loads.