The gradual development of rookie tight end Eric Johnson has been pushed into overdrive.

The seventh-round draft choice will make his first NFL start for the Niners on Sunday against the St. Louis Rams.

That Johnson's first start comes in his second professional game is due to two primary reasons:

--- Regular starter Greg Clark remains sidelined by a troublesome hamstring injury that continues to put his return to the lineup in doubt.

--- Johnson, quite simply, has worked his way into the lineup by becoming the best healthy tight end on the roster.

"We're going to take off the training wheels and let him go," 49ers coach Steve Mariucci said Wednesday afternoon.

The move comes as no surprise. Johnson - who was a star 230-pound receiver at Yale last season, but has added more than 20 pounds to his frame since January to make the position switch - played well in the preseason and appeared to out-perform second-year veteran Justin Swift. But Swift started San Francisco's season-opening win over Atlanta. Johnson, however, actually played more snaps, and he proved he could handle all facets of the job. Johnson blocked well and also was third on the team with four receptions for 34 yards, providing the Niners with a reliable outlet in a passing game that's built around receivers Terrell Owens, J.J. Stokes and Tai Streets.

"He's a tight end that's full of potential," Mariucci said. "He's getting better every week. He works hard. It's all in front of him. He's got a chance to be a heck of a player. We see him as a starting tight end in this league." And come Sunday, that's exactly what Johnson will be.

INJURY UPDATES --- Mariucci said Clark's injury remains a mystery that could keep him out for an extended period. "That probably sums it up," Mariucci said. "Initially, they thought he'd be out two to four weeks. Well, we're past that. He's a little hesitant to really stretch that thing out, get to any full-speed-type running. So, really, it's an unknown."

Could it be something that keeps Clark out for the season?

 "God, I would hope not," Mariucci said. "But I'd be lying to you if I said I wasn't concerned about it. We were hoping to have him back by now, at least in uniform practicing, and I don't know how close he is to doing that."

--- Second-year linebacker Julian Peterson, out since the preseason finale with a high ankle sprain, worked out in shorts on Wednesday, doing some running and working through bag drills. He's listed as doubtful for the Rams game. "We may sit him out this game and this week and get him practiced up for the (Oct. 1) Jets game," Mariucci said.

-- Rookie linebacker Jamie Winborn has returned to practice but is listed as questionable for Sunday's game. … Stokes also is listed as questionable. He participated in the morning walk-through Wednesday, but did not practice in the afternoon.

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