Each time he lines up at safety, Virginia fifth-year senior Marquis Weeks runs through a mental checklist in the moments before the ball is snapped.

He finds the tight end to determine the strong side of the offensive formation -- and therefore whether he will be the strong safety or free safety on this play. He eyes the offense, watching to see if a back or receiver goes in motion, perhaps yelling out to his teammates to change the coverage accordingly. Then the play begins, and Weeks has to quickly figure out what the opposition is trying to do -- again, usually by keying on the tight end -- and how to react appropriately.

It can be a lot to keep straight for any player, but especially so for one only a few months into the job. Weeks shifted to defense in the spring after four years fighting for playing time at tailback.

"The mental aspect of it," he said, "is just totally different than being on offense."

Weeks's transition has gone well so far; his continuing development once the season begins will be one of several factors that determine whether Virginia's talented but inexperienced secondary strengthens or cripples its otherwise imposing defense.

"It's about dependability," Coach Al Groh said. "You can't afford to make mistakes back there."

"It's always in your head," Weeks added. "Not only do the coaches drill it into you, but the defense drills it into you. And plus you're already thinking, 'If I miss a tackle, they get a touchdown.' "

At least Weeks has four years of college football experience to fall back on. Ten of Virginia's top 12 defensive backs are freshmen or sophomores, including all the cornerbacks. At least three of the five true freshmen are expected to play when the 16th-ranked Cavaliers open the season Sept. 4 at Temple.

"That's a group that because of their youthfulness has the potential to make a lot of progress every day," Groh said, "and that certainly has been the case. . . . I've expressed to them, as to all the first-year players, what I'm looking for is a couple of the rookie players . . . to come in and be candidates for playing time in a situation where nobody might have expected that they would do that. . . . There are a few that are really trying to chase down that chance."

Safety Nate Lyles, a big hitter from Chicago, has earned a second-team spot behind Weeks and senior Jermaine Hardy. Cornerback Philip Brown, perhaps the best high school player in Virginia two years ago, plays on the nickel defense alongside redshirt sophomores Tony Franklin and Marcus Hamilton (Centreville). Jamaal Jackson has also impressed at safety.

The newcomers and Hardy's recovery from the knee injuries that plagued him last season appear to have made the defensive backs a far more athletic group than in seasons past.

"We're all fast," Weeks said with a smile. "But I might be the fastest."

Before this year, the Berwyn, Pa., native had to be satisfied with showcasing that speed primarily on special teams. Competing with Alvin Pearman, Wali Lundy and Michael Johnson for carries, Weeks rushed just 63 times the past three seasons, though he averaged 4.8 yards per carry.

But with Jamaine Winborne graduating and Willie Davis's career essentially over because of injuries, the Cavaliers anticipated a hole at safety in 2004. Weeks agreed to try it out. He spent the spring and summer learning with Hardy and the coaching staff.

All along, he prepared with the knowledge that he would have only one chance to get it right.

"You either learn it this year or you don't," Weeks said. "I don't have another year to come back and get better. I have to get better now."

Cavaliers Notes: Lundy, the starting tailback, spent at least a significant portion of Wednesday night's practice on the sideline, wearing a towel on his head instead of a helmet. Asked if Lundy was injured, Groh declined to answer, citing his policy of not discussing injuries. . . .

Blocking tight end Patrick Estes continues to sit out because of migraine headaches. The team is taking a "conservative approach" with the 280-pound senior, Groh said, "to try to get this thing taken care of once and for all."

"I don't have another year. . . . I have to get better now," said fifth-year Virginia senior Marquis Weeks, who is switching from tailback to safety.