ST. LOUIS — St. Louis Rams running back Steven Jackson will not play Sunday against the San Francisco 49ers because of his injured right thigh, and coach Jim Haslett said he won't even make the trip.

Jackson did not practice Friday. By sitting out at San Francisco, he will have missed three of the last four games.

Antonio Pittman and Travis Minor will be the running backs. Pittman, who has been playing through a hamstring injury, has 40 yards on 23 carries in the past two games.

"He needs to break through and get a 100-yard game and show that he's a back who can play in this league for a long time," Haslett said. "I've already told him that he needs to run his butt off to prove that he deserves this opportunity."

Jackson was hurt while landing awkwardly in the fourth quarter of a victory over the Dallas Cowboys on Oct. 19. He has 525 yards rushing, a 4.1-yard average and four touchdowns, and is tied for second on the team with 25 catches.

"I really didn't think I would miss a game, let alone two or three," Jackson said. "For it to linger like this, it really begins to make you wonder what's really going on."

The Rams (2-7) haven't changed their diagnosis of a strained thigh. Jackson said doctors have told him two MRI exams have revealed mostly swelling and inflammation.

Haslett said cornerback Tye Hill (knee) out for a fifth straight game following arthroscopic knee surgery, also would not make the trip.