It's not slander. Wilks literally said he was the reason he got cut.
“This is a tough business … Sitting in this seat, it was a tough decision on my part, but something I felt like was the best move for the team moving forward."
Now that may or may not have been coach speak but he definitely did say it when he didn't have to.
And I don't think they were wrong to move on from him. He was ass for the Panthers and ultimately it worked out extremely well for him in the end. He's turned it around in Tampa and has firmly established himself as one of the very best QBs in the league, that obviously doesn't happen in Carolina.