The Swansea match yesterday made that point painfully apparent. Rooney should not be playing up front as a #9--he plays much better as an off-striker in a #10 role. He has never been blazing fast, but even then, he's lost a step recently. His first touch has always been a bit dodgy, and has cost him some scoring opportunities playing up front.
In addition to Martial, I hope that they can go after a player like QPR's Charlie Austin. He's available for 15M pounds and is a proven striker in the EPL. Having him would provide some extra depth and quality.