1. A big part of the poor execution is the poor play design and poor play calling. He is not putting the players in a positon to perform. Case in point is designing and calling plays where you need 1st downs but you have your best players run routes as decoys in no position to get the 1st down. (Moulds in NE, Evans vs Pats)
2. The 2nd part of poor execution of the offense is the terrible play of the OL. Most NFL teams have an inadequate player or 2 on their line, but the coaches come up with some scheme so they can at least compete each week. The Pats are starting 2 street free agents on their OL. How is it that they don;t completely melt down each week? The answer is obviously the coaching and MM is responsible for that. McNally looks to be totally overrated.
3. If Clements sucks, that's on MM as well because he hired him. HOwever, after taking play calling from Clements, nothing really changed on the field. The Bills still look good for the 1st drive with their scripted plays, but are totally lost after that. Running Willis only 8 times in the snow against NE is inexcusable.