You are exaggerating his ineffectiveness. At no point did he comes close to the horribleness of J.P. Losman. Even in the Cleveland game he wasn't as erratic as J.P. is on his best day and came back strong in the second half. If the offense was tailored around Jackson and Lynch, I don't see any reason why he can't be an adequate starter with a veteran backing him up.
Honestly if you take out the three game AFC East stretch plus the first half of Cleveland I'm sure that the stats are going to look all that bad. He didn't look even as close to as bad as you try to claim he did in our wins and he didn't look as bad in the late losses as he did in the middle ones.
Center, Defensive End, Tight End, Wide Receiver, Guard, Fullback, Linebacker, Safety, Defensive tackle are all higher priorities in my book.