IT HAS BEEN quite a metamorphosis for Penn State quarterback Daryll Clark, who in 21 months has gone from mostly runner to almost exclusively passer.

Oh, there is always the possibility that sometime this season, by chance or design, the 6-3, 232-pound Clark might take off for the sort of long gain he once routinely produced. But it is more likely that Clark's mobility will be restricted to modest rollouts to evade opposing pass rushers who no longer consider him much of a threat to run. Clark is evidently too valuable to the fifth-ranked Nittany Lions to risk the sort of hits that led to his concussion in the Ohio State game last season.

How the fifth-year senior from Youngstown, Ohio, is utilized in tomorrow night's nationally televised Big Ten Conference opener against Iowa in Beaver Stadium should provide at least a clue as to what restrictions will be placed upon him for the remainder of the season.

"We're trying to win the football game," Penn State coach Joe Paterno said when asked whether Clark ever again will showcase his full complement of talents. "If that means we have to run Clark, we'll probably run him.

"But he's awfully valuable to this football team, and I don't want to get careless and start making him a single-wing tailback. He's a quarterback. We're certainly not looking forward to running him much. But if we had to, we could."

If JoePa's take sounds a bit vague, Clark understands that his days of playing quarterback the punishing, physical way he once did are over. All-Big Ten QBs are to be protected like gold at Fort Knox, especially if their backups are a couple of freshmen with virtually no game experience.

Put it this way: If Daryll Clark is not the Big Ten's best player, he is arguably its most irreplaceable.

"I don't really want to say it limits me," Clark said of his conversion to a more conventional pocket passer. "Eventually, we're going to start to run the quarterback, just to give the defense we're playing against a different look.

"But as long as the run blocking is going well, there's no need to use me as a runner. That doesn't mean there won't be some times when I have to scramble and make some plays with my feet."

If Penn State does choose to expand its playbook to use the quarterback on designed running plays, that role likely will be filled by true freshman Kevin Newsome, a 6-2, 220-pounder who greatly resembles the player Clark was before his coming-out party in the Alamo Bowl against Texas A & M on Dec. 29, 2007.

Two-year starter Anthony Morelli, a pure pocket passer, was having difficulty getting the Nits going after they fell behind, 14-0, in the first quarter. That opened the door for a change-of-pace quarterback to help swing the momentum. Clark, who had played sparingly throughout the season, provided a much-needed spark, rushing for 50 yards on only six carries, including an 11-yard touchdown.

Heading into preseason camp in 2008, the perception was that the quarterback competition would feature Clark, perceived as the better runner, and Pat Devlin, who supposedly had the NFL-caliber arm. Clark won, and quickly showed he was a dual threat, passing for 2,592 yards and 19 touchdowns and rushing for 282 yards and 10 TDs. Opposing defenders had to be constantly aware of Clark's proclivity for scooting up the middle on quarterback draws or even sliding down the line of scrimmage on option plays.

Devlin transferred to Delaware last December, and Clark entered this season backed only by the skilled but very raw Newsome and redshirt freshman Matt McGloin. Concerns about Clark's health rose almost immediately, when he twice had his helmet knocked off in the Akron game, on more or less the same play that left him dazed and confused against Ohio State in 2008.

Clark insists he's all right with the current arrangement because "we have a great running back" in Evan Royster, whose duties have expanded even as Clark's have been constricted. Royster occasionally will line up as a slot receiver, giving defenses a different look and keeping them guessing.

Still, Clark said, only about 70 percent of the Penn State playbook has been utilized through the first three games. With Iowa - which dealt the Nits their only regular-season loss a year ago - providing the opposition in the Big Ten opener, some new plays could appear.

Such as Clark taking off on a designed end sweep on a crucial third-and-short?

"You never know," Clark said. "We've shown a type of spread. At times we've been in the power-I. But we really haven't revealed that much, I don't think. We have a collection of plays we only bring out if they're going to work against the defense we're playing."

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* Clark still blames himself for last season's 24-23 loss at Iowa, and not only because of the late interception that opened the door for the Hawkeyes' winning drive.