He was a huge part of the problem. That's why they fired him in mid-season! That doesn't normally happen you know, firing an assistant coach in the middle of the season.
He literally refused to alter his game plan to match the reality that he didn't have Philip Rivers, but two very inexperienced QBs and a piss-poor OL that couldn't pass protect for them. So, of course, he dials up an offense with 4 and 5 WR sets and single blocking on the outside against elite pass-rushers, etc.
The results speak for themselves. Trevor got hurt again behind this OL and it ruined his career.