Mariano Rivera has a blood clot in his calf:

Anyway, Rivera says that, while the clot “scared” him, it has already been dealt with and that, as soon as his leg loosens up some and he can get a better range of motion with it, he should be slated for surgery. As reported yesterday, this should not impact his recovery timetable at all.

It should scare him. Blood clots in legs sometimes travel and end up killing people. It’s a good thing they caught it so it could be dissolved.