the latest report i've seen out of buffalo is that a-train will be starting this week and most likely next week as well and after that, its up in the air depending how well/quickly willis recovers.. Apparently McGahee was in the training facility this week, walking upright(couldn't do last week) and looked much better, having a lil "pep in his step" so to speak. I'd assume that despite the Bills season going down the crapper, if Willis can play, he will play. Its not like the portis situation, where he has the opportunity to get the rest of his injuries healed up. McGahee's type of injury usually keeps players out between 1-5 weeks depending on the severity. One broken rib and two cracked ribs in his case should keep him out somewhere between 3-5 weeks, most likely having him back by the time playoffs roll around.
Nothing is definate of course, but that is just my opinion based off the latest i've seen regarding McGahee. Having both willis and a-train is the safest bet, if getting willis is possible without hurting your starting roster, go for it.