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http://www.comicbookresources.com/news/newsitem.cgi?id=10527> Preview of Mighty Avengers #3 over at CBR.
In the last page it looks like blood is dripping from the Sentry's fist. Now, either he punched Ultron so hard he was able to somehow get to Stark underneath assuming Stark is still in there or he busted his hand when hitting it.
Either that or it is just red cloth handing behind that looks like it might be coming out of the Sentry's fist. I wonder if that confusion was the intention of the artist?