Booggie
Fighting a army of vampires while defending against giant stone golems is much different than a one on one fight. Morpheus is the definition of prep time. He was on their territory being seiged at all sides, no way to reposition as he couldn't apparate at all to even find a vantage point. It would have been unrealistic to have him pull out a overpowered spell out of his a** when I have shown him to be the most dominate when he has even the smallest amount of time to prepare. He does not let this slide and get his get back fairly quick. From a strategic outlook retreating there was a better option so he can take them out more sufficiently. In a couple chapters you will see his plan and what actions he takes.