Nathan Baker was taken off the pitch on a stretcher in the first half after being hit in the head with a shot off of the foot of Arsenal's Serge Gnabry. Baker was on the ground being attended to for more than five minutes. At first, the reaction on twitter was of the standard "Well, Baker with another head injury" variety, but the jokes became more subdued when it became apparent that Baker wasn't getting up any time soon.
The immediate worry is, of course, that Baker has suffered a concussion. But generally, a stretcher is not involved in that sort of an issue, so it's really hard to say what might have happened. We will, of course, have updates as the emerge.
The overwhelming concern should be that Baker is doing alright and that the moment simply looked more scary than it actually was.
UPDATE: Baker apparently has a mild concussion. We've got more here.