Thursday night in Kansas City, Mariano Rivera was shagging fly balls during the Yankees' pregame activities when he apparently suffered a knee injury, and was placed on a cart and taken from the field.

Rivera reportedly injured his knee when he made contact with the outfield fence, though it's not clear how severe the contact. Actually, the degree of the contact is irrelevant now; all that's important is the severity of the injury. Considering that Rivera was attended by medical staff and then lifted onto a John Deere groundskeeper's cart, we might guess Rivera's injury was not minor ... but again, it's just a guess. Perhaps he suffered a mere contusion and he'll be pitching again next next week.

Here's Bill Ripken on MLB Network:

We can't speculate on what's happening. But if you read body language, and everything that we just saw, they were very careful with him, putting him in the vehicle. There's no doubt about that. And that look on Mariano Rivera's face ... was pretty telling to me.

Not to speculate or anything. But if Rivera's down for a while, the Yankees will almost certainly turn to Dave Robertson -- who hasn't given up a run this season -- as their new closer, with everyone else moving up a slot in the bullpen pecking order.

Update: We've now seen film of the incident, via MLB Network. Rivera did not crash into the fence. Rather, he was reaching to catch a fly ball and simply crumpled to the ground, immediately curling up and clutching his right knee.