Any clause preventing him from playing against us would be illegal. Chelsea reportedly had a clause in Courtois's loan agreement with Atletico and when they drew each other in the CL Chelsea were forced to let him play.Yeah, some sort of 2nd medical clause I would imagine is a given, I wonder if there is also a clause preventing him from playing against us in the , unlikely, event of Liverpool encountering Leipzig in the CL this season
edit: Just double checking and the Courtois clause supposedly forced Atletico to pay Chelsea an additional fee if he played vs them but still, UEFA said it couldn't be enforced.