Here's the thing with Matthews. When he's been healthy he has consistently been the best player on defense (at OLB). He also makes other players better, as evidenced by Nick Perry's play when Clay is opposite him.
The argument can legitimately be made that he's just so-so playing ILB. While that may be true he still played hard when he was in there the past two seasons, and he shored up a miserably weak ILB group when they moved him in there.
Who should be held responsible for the sorry state of the ILB position at the time? It's not Clay's fault that TT rolled the dice on the ILB position during 2014. In 2015 he added an average guy to the mix but was still short with Clay being the only viable alternative. So he took one for the team again last season and again played played out of position at ILB. This year he added another average ILB, thus releasing Clay to play at his regular position.
But this season, despite being back at the position where he belongs, he has been playing injured. First the chronic hamstring and then the shoulder separation. Despite that he has continued to play because he was needed. He even sucked it up and has played the past few games with a very painful shoulder injury that virtually renders him one-armed. A drop-off in performance has to be expected, especially with a significantly painful shoulder separation. It's not like he's healthy yet his performances continued to be sub-par.
Did Nelson get asked to take a pay cut because he tore his ACL last preseason? Of course not. Injuries are a part of the game and Matthews has been injured, unfortunately, but he still plays. In pain. Doing the best he can under the (injury) circumstances - his as well as others. Had he been out with a torn ACL all season long would people still be calling him out and suggesting he take a pay cut because the injury affected his play? I sure hope not.