My take by description of the event without the benefit of seeing it, most likely thing is it's a day to day sprain. Likely to heal quickly, but can't practice on it and risk further injury.
Kolb probably knew it the moment it happened, and threw his helmet in disgust because the praise he's given EJ over the last few days is not lip service. He knows any advantage he has over Manuel because of his experience is tenuous. Kolb, by reports, had his best day of practice yesterday, and any momentum that he he'd gained is now probably lost. I'm sure he realizes he is not the favorite in this close QB competition, and regardless of what he says, I'm sure it's weighing on him.
If it was something he could practice through without risk of further injury, I'd hope someone in his situation would work through it. If not, there is no actual competition.