PITTSBURGH — Running back Patrick Cobbs, an undrafted rookie free agent who led the New England Patriots in rushing during the preseason, was traded Friday to the Pittsburgh Steelers for a conditional 2007 draft pick.

The Steelers have been seeking another running back as former starter Duce Staley struggled during training camp after missing all but a handful of games last season following knee surgery.

The 5-foot-8, 210-pound Cobbs was a surprise for the Patriots this summer, rushing for 188 yards on 38 carries while averaging 4.9 yards per carry. He had a 92-yard game against the Redskins in a 41-0 Patriots victory, though he was playing against backups.

Adding Cobbs could mean that neither Staley nor seventh-round draft choice Cedric Humes of Virginia Tech will make the Steelers' 53-man roster, which must be determined by 6 p.m. Saturday.

Cobbs is a power runner much like Staley, and it seems unlikely that the Steelers would keep both backs — plus third-down back Verron Haynes — as backups to starter Willie Parker, who signed a $13.6 million, four-year contract extension on Thursday.

Cobbs wasn't invited to the NFL combine in Indianapolis in February despite having led NCAA Division I-A with a 152.7 yards per game average as a junior at North Texas in 2003. He ran for 1,680 yards and 19 touchdowns that season, then missed the 2004 season after tearing a left knee ligament.

He returned to gain 1,154 yards on 231 yards for six touchdowns last season. After not being drafted, he was planning to sign with Montreal of the Canadian Football League, only to catch on with the Patriots after they invited him to their rookie minicamp.

The lack of height that likely kept Cobbs from being drafted also may have dissuaded a top-tier college from signing him out of high school, even after he led Oklahoma Class 4-A rushers with 2,354 yards and 32 touchdowns at Tecumseh High School.

The Steelers were counting on Staley, the starting running back in 2004 until he injured a hamstring after seven games, to take over retired Jerome Bettis' role as a short-yard specialist. They also expected to use him in late-game situations when the team tries to wind down the clock with a lead.

But Staley has looked slow in hitting the line of scrimmage throughout camp, and he has not shown the straight-ahead bursts or shiftiness that allowed him to average 101 yards per game through seven games before he was hurt in 2004.

Because Staley ran poorly in their first three exhibition games, averaging 2 yards per carry, he was given 19 carries in a 15-13 loss to Carolina on Thursday. He responded with only 46 yards and had no gain longer than 6 yards. He finished exhibition play with 92 yards on 42 carries, a 2.2 average.

"He needed to run an extended period of time to see if he could sustain it," coach Bill Cowher said.