TEMPE, Ariz. (AP) - Carson Palmer was going through the NFL's concussion protocol on Monday and Arizona coach Bruce Arians said he wasn't ruling out the quarterback for Thursday night's game at San Francisco.

The loss of Palmer would be another blow to an Arizona team that has stumbled to a 1-3 start after opening the season with the highest of expectations.

Palmer's head slammed to the turf on a sack by Aaron Donald late in the Cardinals' 17-13 home loss to the Los Angeles Rams on Sunday.

Arians said a decision on whether Palmer will play probably won't be made until "probably about the time we get on the plane" Wednesday. If Palmer can't go, the job falls to Drew Stanton, who re-signed this offseason to return as the Cardinals' backup.

PHOTOS: Rams vs. Cardinals