This is why the Steelers drafted Rashard Mendenhall in April.

They drafted a running back in the first round for the first time in 19 years not so much because they needed a younger back, or because they wanted someone to split time with Willie Parker, or because they did not think Parker would be the same after his fibula was broken in the 15th game last season.

They drafted Mendenhall in case, as coach Mike Tomlin might put it, the wheels came off Parker, and Sunday one of them did.

Mendenhall, who did not have a run from scrimmage in the past two games, will make his first NFL start Monday night against the Baltimore Ravens because Parker has a sprained left knee that likely will keep him out for at least the next two games.

"Ready or not, huh?" Tomlin said at his news conference yesterday. "No, Rashard is ready. He is a talented guy and he has been practicing well. He gets his opportunity, and I think that is what team football is all about."

It has been the year of the rookie running back early in the NFL season. Three rookies are among the league's top nine rushers: Chicago's Matt Forte (304 yards), Tennessee's Chris Johnson (276) and Oakland's Darren McFadden (252).

"A lot of his peers around the league, they have been getting opportunities, partly because they do not have Willie Parker playing in front of them," Tomlin said. "He gets his opportunity. I'm sure that he is excited and I know that our team is excited to see what he is capable of doing. What better way to start than to do it on Monday night football against the Baltimore Ravens?"

Losing Parker wasn't the only bad news Tomlin delivered yesterday. Nose tackle Casey Hampton, like Parker a selection for the Pro Bowl last season, will miss the Monday night game with a groin injury that forced him from the game Sunday in Philadelphia.

Hampton has a proven replacement in Chris Hoke. Mendenhall is unproven in the pros. He looked good running in the preseason when he wasn't losing fumbles, but he has done little in three games. He carried 10 times for 28 yards in the opening rout of Houston but has no carries since. He did not play on offense in Cleveland and did not play in Philadelphia until after Parker left late in the game with an injury that was undiagnosed until the team returned to Pittsburgh.

Mendenhall dropped the first pass thrown to him against the Eagles, caught another and ran well with it, picking up 11 yards. He has returned six kickoffs for a 19.2-yard average with a long of 27 yards and he let another bounce and roll out of bounds at the 2.

It has been an inauspicious start for the rookie running back, but he has the pedigree, which is why the Steelers drafted him in the first round.

His first start will come against the NFL's top-rated defense and fifth-best run defense. Baltimore has yielded an average of 161.5 yards a game and 71.5 yards on the ground in two victories at home against Cincinnati and Cleveland.

Tomlin said Mendenhall will be the primary ball-carrier with third-down back Mewelde Moore and fullback/halfback Carey Davis backing him up. It's likely the Steelers will re-sign Gary Russell to the 53-man roster as well. Russell was released Saturday to make room for special-teams player Patrick Bailey on the roster. The Steelers signed Russell to their practice squad this week.

Tomlin would not go into detail about what ligament Parker damaged, but a sprain means a tear to one of them. Tomlin said it happened near the end of the 15-6 loss, in which Parker carried 13 times for 20 yards. He was the NFL's third-leading rusher after two games and has 263 yards and a 4.0 average with three touchdowns, all in the opener.

"He came out, and we knew he was uncomfortable, but we didn't get the information until yesterday when we were able to run some tests," Tomlin said.

Tomlin reasoned that Mendenhall had no carries the past two games based on situations.

"The other night we had a tough time getting any rhythm going offensively. I think Willie only had 12-13 carries himself. That is just the way that the game unfolded."