Thibaut Courtois continues to be one of the best goalkeepers in the world while under contract with Chelsea and playing for someone else. He will probably replace Petr Cech eventually, but it's not clear when that's going to happen. Chelsea could always bring Courtois back from Atlético Madrid next season and let him compete with Cech for a starting role, but Courtois doesn't seem too keep on that idea. His reasoning makes a lot of sense:
"The two of us want to play everything. Nobody will be happy to play in only the League Cup or FA Cup. Things are unclear at the moment ... I have the biggest respect for Petr Cech. He is one of the best goalkeepers in the world so I think putting him and me in the same team is not the healthiest thing in the world."
There was probably something lost in translation with that last little bit, but his point stands. Cech and Courtois are undoubtedly two of the five-to-ten best goalkeepers in the world, and neither of them wants to play on the same team as the other. They're both good enough to be first choice for Champions League contenders -- as we've seen this season -- and one shouldn't be backing up the other.
So this puts Chelsea in a bit of a tricky situation, though they could always delay that decision for another year. Courtois would probably be happy to sign a big money extension if it means that he gets to spend another year or two at Atlético Madrid, while Cech would probably be happy with starting for Chelsea for another year or two before moving on. It costs them money, but Chelsea have plenty of that.
Chelsea can probably keep compromising for at least one more season, but Courtois' comments are a reminder that they need to offload one of their keepers eventually or risk damaging their relationship with one of them irreparably.