TEMPE, Ariz. -- Arizona's top draft pick Patrick Peterson could move into a starting role after cornerback Greg Toler was lost for the season with a knee injury.

Toler tore his left ACL in Saturday night's preseason loss to San Diego and was placed on injured reserve Monday. The Cardinals could wind up with an exceedingly young set of cornerbacks, with second-year pro A.J. Jefferson at one slot and Peterson -- the No. 5 pick in the draft -- at the other.

Whisenhunt likes to bring along rookies slowly, even ones as touted as Peterson, but circumstances may have forced his hand.

"You get to a point where your young guys have got to play, they've got to step up," Whisenhunt said after practice Monday. "I definitely feel good about A.J. and his progress. He's been with us for a year, going into his second. And Patrick, there's no question he's a talented young player. He just needs a little time. If we're playing with those two guys, then that's because they've earned it and I'm OK with that."

The Cardinals, of course, sent big-play cornerback Dominique Rodgers-Cromartie to Philadelphia, along with a second-round draft pick, in exchange for quarterback Kevin Kolb.

Jefferson, on the Arizona practice squad out of Fresno State last season, has been one of the big surprises for the Cardinals' training camp, laying claim to Rodgers-Cromartie's old job.

"I'm never satisfied as far as how things are coming," Jefferson said, "not until we get into the season and start doing things solid every down. But things are going good and I'm just going to get better every day."

He appears to have one cornerback spot sewn up, with Peterson and six-year NFL veteran Richard Marshall the candidates for the other spot. Marshall, who started every game for Carolina the past two seasons, was signed as a free agent.

Late Monday, the Cardinals added another cornerback to their roster when they signed Fred Bennett, who was released by Cincinnati on Saturday. Bennett split last season with the Bengals and San Diego after playing for Houston his first three seasons in the NFL.

Peterson said he and Marshall were splitting time with the first unit Monday.

"I believe I'm ready," Peterson said, "but the decision ultimately is going to be the coaches', the defensive staff as a whole. The head coach may have input, I don't know, but like I said I have to continue working hard each and every day."

The former LSU star's natural ability already has been on display. Against San Diego, Peterson stepped in front of Antonio Gates and intercepted Philip Rivers' pass, then returned it through traffic 34 yards for a touchdown.

"That's what I'm supposed to do, I'm supposed to make interceptions," he said. "But at the same time I'm definitely happy that it came early, to kind of get it out of the way. I'm definitely happy to have my first interception for a pick six."

Peterson said he and Jefferson could "lean on each other" for support if they wind up being the starters.

"Both of us are young, but we have veterans behind us," Peterson said. "Those guys are definitely coaching us up."

In addition to Marshall, Arizona soon should have the services of backup cornerback Michael Adams, who has been sidelined with a sprained right knee. Adams went through a limited practice Monday and said he was planning to be ready for the regular-season opener Sept. 11 at home against Carolina.

"It hurts us to lose `DT," Adams said, "but I think we're still pretty deep at corner."

Toler, drafted two years ago out of tiny Saint Paul's, Va., had looked increasingly comfortable in the starting role.

"Man, that breaks my heart," Jefferson said. "I'm praying for him for a speedy recovery."

It's the second significant season-ending knee injury for Arizona. Rookie running back Ryan Williams ruptured his right patella tendon in the second game of the preseason. He also was placed on injured reserve on Monday.