It was cramps, they were rubbing down his calves on the sidelines and giving him fluids. I'd guess he didn't properly hydrate prior to the game or he was so busy during the game he was forgetting to hydrate. It can happen in no time if you're not taking care of yourself.
The key why this isn't an actual injury: They never took him into the tent and the doctor was never with him, was always just the training staff which generally means minor issues or cramps.