Though someone going henge may make little sense, it makes very little sense to me how a person you saw in need of life threatening surgery in the morning shows up the afternoon fighting with a new technique. Learning a technique in his injured state? Makes about as much sense as Gai going henge.

My guess is it's Sakura. What better way to put her in a pivotal role (FINALLY). Plus she fully knows what Lee looks like, wants to help, and knows Naruto would stay behind if he saw her fighting.

Yep, it's totally out there, but it makes a lot more sense than Lee to me.