Yuu is a pretty emotionally stunted/fragile guy. He doesn't know how to deal with situations like this. The karate prick practically jumped him and he was flooded with a bunch of different thoughts and feelings just trying to piece together what was even happening. He was already weirded out by the situation with Shougo and he thought this was a step to reconcile with him, but then he had the carpet pulled out from under him and now he feels Shougo's reputation is somehow at stake because of his failure. To me it didn't seem like he forgot how to fight or anything, just that he was flustered with the situation and got injured from a cheap shot.