Ramires rarely plays a holding role for Chelsea though. He's either played in central midfield (ahead of the 2 DMCs) or on the right wing for defensive cover. Mikel is their sub for closing out games, he's slots in beside Matic with Oscar or Cesc ahead of them.
imo, Mourinho's ideal midfield is basically someone really fucking big who sits (Matic/Mikel), a creative player who stays deep (Fabregas), and someone ahead of them who runs a lot (Oscar/Khedira/Ramires). Oscar would offer guile, khedira would offer power.
Plenty of reasons for the transfer to happen, and they also have an excellent relationship with each other.