I think you have to look at who is playing versus who, most of the starters, is not playing. You're seeing 2nd, 3rd, and practice squad players who want to make a name for themselves. Many of the starters have already gotten their pay days and its easy for them to fold up their tents and quit when things aren't going good. CeeDee is the only one still fighting, and I have lot of respect for him because of it.
So I really don't think anyone is playing harder for one QB or the other, the players still on the field are just taking advantage of an opportunity they probably wouldn't get under more ideal situations, and they're busting their butts. This season has looked a lot like an extended pre season where most of the starters wouldn't have been playing anyway.
There's nothing magical that MM is doing to motivate the players, it's nothing more than a group of players taking advantage of an opportunity and they have been unexpectedly successful. Sure, it's been against equal or lesser competition, but so what, they're playing hard and that's what matters.