Mendenhall accounted for 21 yards on 12 carries with a pair of fumbles. He also dropped a pass and scampered out of bounds late in the game as the Cardinals were angling to burn time off the clock.
"Rashard had a very tough day, and he can't play that way," Arians said, per Kent Somers of The Arizona Republic. "He can't run out of bounds. He's a veteran. And to run out of bounds at the end of the game, that was probably the worst mistake he made all day."
Arians openly suggested his so-called featured back needs to do more during the week to be ready for games.
"We need to practice him a little bit harder," Arians said. "I think we've been too easy on him in practice because of his injuries, and we need to hone him up more because he's not gaining anything."
Mendenhall has sneaky speed, but it was rookie Andre Ellington who looked like the better option on Sunday.
Arians obviously has an affinity for Mendenhall, having brought him to Arizona after years together in Pittsburgh. As Somers points out, it's unusual to hear a coach suggesting a veteran needs to practice more, but Mendenhall isn't on a long leash. If someone else steps up, his role in the backfield is on thin ice.