In my opinion it was more the lack of execution than the play calling although Garrett did get way too conservative at times. His conservative play calling in the closing minutes vs NE in 2011 cost the Cowboys that game. The biggest knock on Garrett has been his inability to manage games which is one of the main reasons he won't be calling the plays. Jerry wants to free him up so he can concentrate on managing games better. There's been issues when and when not to call timeouts no bigger issue than in the infamous icing the kicker game vs AZ in 2011. There's been a lot of confusion with players not knowing when to be on the field or where to lineup. We saw players not hustling in the closing minutes vs the Ravens last season which cost the Cowboys valuable time in which they could have run another play that could have made the FG attempt easier.
Garrett's game management has been under a lot of scrutiny the past 2 seasons. With the effort being made to try and help him improve in that area he doesn't have much wiggle room left. Jerry has made every effort to try and make it workable with Garrett as the HC. The way the NFL is today it's hard to put teams away especially when you're a mediocre team. The Cowboys biggest margin of victory last season was only 15 points. It's going to be a dogfight practically every week this season so managing games is going to be critical to the Cowboys success.