First off you are correct, he can't force the coaches to do anything. But he can hire coaches who make changes when things aren't working, more about the coaching below.
About Willis McGahee, why do you waste a first round draft choice on him if he is going to sit out one year and then sit on the bench the second year? Wouldn't a quality offensive or defensive lineman be a much better use of the pick?
Lee Evens, why again did the Bills need a receiver? Oh yeah, Donahoe blew a second round pick, 36 overall, on Josh Reed.
Speaking of TREMENDOUS draft picks, what about the using the 4th pick in the 2002 draft on Mike Williams? That seems to be working out very nicely.
Didn't Mr. Donahoe use a few second round draft choices on defensive linemen, Kelsay and Denney? Where is the pass rush? The reason the secondary is getting torched is because they are blitzing all the time because the front 4 can not get any pressure on the QB.
Just because there are a lot of "big names" on the roster it doesn't mean TD is doing his job. His job is also hiring competent head coaches, Gregg Williams was a complete failure (wasted 3 years), and the jury is still out on Mike Mularkey (although its looking like more of the same garbage).