Yes 2013 was one of, if not his worst. And while subjective, 2014 was one of his better, but if we're using QBR, it was his 3rd best.
So, it's not like Eli was hot garbage before 2014. And I'm not sure why he was so bad in 2013, injury, bad year, system change, etc, but the sample size isn't enough to contribute it specifically to a QB coach.
Was 14 his first year with Beckham?