Franks isn't Mullen's guy so I don't think it is going to be hard to make a change in this case. Unlike the last 2 dudes, Mullen is at least good at working around the gaps with the inherited guy in the meantime. But you can almost feel his frustration every time you see him coaching up Franks after yet another missed opportunity.
I think Mullen ultimately chose Franks as the starter this year to see if he could mold him into the sum of his raw talents. Unfortunately, Franks is another Rex Grossman or Doug Johnson type who has massive physical skills but lacks the mental/emotional focus necessary to consistently elevate his game in big situations which, in the end, is what defines champions.