I have to believe that Grier's failure to properly address the O-line to protect Tua, and work in McDaniel's system puts him on the hot seat. I think this falls on McDaniel, as well, but there isn't the type of HC/GM marriage in Miami like there is in Buffalo. Tua's biggest flaw, IMO, is his poor ability to work off-script, and keep his eyes downfield when facing pressure. Add in his concussion history, and Grier's failure in this regard is truly unforgivable. And Tua's contract must certainly have raised Ross's blood pressure when half the league was calling for Tua's retirement, after his last concussion.
I'll say this also-- go onto one of the Dolphins fan forums, and you'll learn pretty quick that Grier is utterly despised by the fanbase.