I don't think this can be particularly serious, since the defender managed to walk off the pitch, but the timing of the substitution is slightly worrisome. You don't pull players off in first half stoppage time unless they're completely unable to play, and that's what happened when Roberto di Matteo put Gary Cahill on just before the break. We'll know more about just how severe it is in the next couple of days - I assume that means that he'll have a scan but that his ankle's too swollen to manage that right now.
I'm not going to pretend to be able to diagnose this from afar, but obviously I hope this isn't too bad. David Luiz is a very, very good player and losing him for any length of time would be pretty awful. The Drogba injury doesn't sound as bad, but his absence would hurt the club too. Get well soon, both of you.