We've seen this plenty of times, the jaguars this year, ravens in 2012 and various other collapses over the decade.
I think you definitely give him a year to rebound and see if he can cook up some magic after a down year, great coaches do rebound after down years (Tomlin, Harbaugh, Reid, Bellicheck etc) Sirianni reminds me a lot of harbaugh, without good coordinators he's a great people guy and motivational guy but not an Xs and Os type of dude.
The fact matt patricia was the DC is legitimately stupid though. Should've been the offensive coordinator.