It's a difficult one because the better he does, the more difficult it will be to keep him. Possibly best to try and do a deal now whilst he's in the middle of goal drought.
If he suddenly starts firing again and banging in goals for fun it could well add a few more million on to Chelsea's asking price.
If he suddenly starts firing again and banging in goals for fun it could well add a few more million on to Chelsea's asking price.