Here is the issue with Guarin, and analrapist touched on this, as have others. He is not disciplined or good enough defensively to play as a CDM in a 3 man midfield... he is also not creative, intelligent or unselfish enough to play as a CAM or as the more attacking member of a 3 man midfield. His only role can be that of a pure CM in a 3 man midfield, and we already have kovacic for that.
If we get nainggolan or taider, i'd rather see a nainggolan/taider ----- kovacic ------ alvarez midfield and have guarin on the bench.