It's an unconfirmed report, but Kristof Terreur is usually on point with news related to Chelsea's Belgians, and he's probably trustworthy in this instance as well. It should also be stressed that the severity of said injury is not known yet. It could be a minor thing, like Courtois suffered in warm-up before Swansea City. Or it could be severe. For now, this is all we know.
Thibaut Courtois suffered a knee injury on training yesterday. Full extent of injury yet unknown. #cfc— Kristof Terreur (@HLNinEngeland) September 10, 2015
Terreur's full report in HLN states that Chelsea are concerned the injury might be severe enough to keep Courtois out of action for more than just the weekend, though with games now coming thick and fast (one every three days or so until next month's international break), the actual length of the injury may still not be too great. Again, we just don't know at this point.
The report goes on to point out that Courtois has yet to suffer a major injury in his career. That's certainly part of the reason he's become one of the world's best over the last few years and part of the reason he managed to establish himself as Petr Cech's successor last season. Now Cech is gone. Fortunately, Asmir Begovic is more than a capable deputy.
Updates as we see, hear, get them...