Suh is a big loss; there is no two ways about it. That being said, I think the D would have managed to be productive (as in top 10) had Mayhew not gone full retard and done nothing to prevent the utter destruction of our D-Line.
Losing Suh due to fucking up his contract so badly you cant even franchise him (aka the rule made so teams like the lions can keep players like Suh) is one thing, but Mayhew took it a step further and did the following:
1) Declined the (cap friendly) team option of a talented (2nd best) DT in Fairley in hopes to 'motivate him' which in turn, pissed him off and is the main reason he left in the offseason.
2) Totally ignored both Mosely and Fluellen (3rd/4th best DT) due to trying to save money for Suh.
3) Lets a fairly productive Pass Rusher in Johnson go to the Bucs for free, due to not putting the correct RFA tender on him...He wasnt going to be a starter here, but Det currently has 4 sacks this year...which is brutal.
4) Passes on the top DT (A Donald) in the 2014 draft for a drop prone TE, which by all indications is a bust. Meanwhile Donald went on to have a dominant rookie season w/ 7.5 sacks, and already had 3.5 this year....
He does all of this after forming the entire defense (personel/scheme) around getting pressure on the QB! Being able to pressure the QB with a 4 man rush was a HUGE advantage to the Lions, and was why they were successful...but he seemingly forgot about that and decided to take a shit on that plan. Well I hope youre fucking Happy Mayhew - you just fucked up your '7 year plan' in one offseason you damn moron