Ryan Bertrand
Bretrand is a strange case; he has impressed greatly wherever he has been on loan and even has experience of playing in a Champions League final. However, if he were to stay in west London he would only be used as an understudy to somebody else as it appears that Jose Mourinho does not trust him to be a first team regular.
Currently with Aston Villa until May, he would be a valuable asset to another Premier League side, earning Chelsea a substantial fee, or he could be used in part exchange with somebody for one of their own stars to go the other way.