Already without his No. 1 offensive stalwart in tailback Silas Redd - he transferred to Southern California on Tuesday - O'Brien learned this weekend that Brown, the leading returning receiver from 2011, had accepted an offer from Oklahoma. Brown is the only senior among the nine players who transferred.
Under rules revised by the NCAA to accommodate Penn State players, all nine are eligible to play for their new schools this season.
Brown, who is from Wilmington, caught 35 passes for 517 yards and a touchdown last season. An inviting target, given his 6-foot-3, 210-pound size, Brown also was Penn State's most experienced punt returner.
With Brown's departure, Penn State has lost 14 scholarship players, including four for disciplinary reasons and one for a medical issue, since O'Brien took over in January.
In a radio interview last week on the "Goon Show," a Penn State football-themed program, O'Brien said he respected the rights of some players to leave but added, "The core of this team stuck together.
"You talk about different positions that have left, but nobody talks about the kids that stayed. We've got a lot of good football players in the program," he said.
Still, O'Brien, who brought a wealth of knowledge from the New England Patriots, where he was offensive coordinator, has to come up with new starters at two key skill positions for an attack that underperformed last season. Redd and Brown combined to generate more than 40 percent of the yards gained in 2011 by the Nittany Lions.
Sophomore Bill Belton, a Winslow Township High School graduate who has never played a down at running back last season, is the No. 1 tailback for now. The five players listed at the wide receiver positions with Brown on the post-spring depth chart have 17 career receptions combined.
The inexperience puts added pressure on senior Matt McGloin, who was designated by O'Brien as his No. 1 quarterback after spring practice ended.
In addition to Redd and Brown, the offense lost another major contributor in kicker Anthony Fera, who moved on to Texas. Fera handled extra points, field goals, punts, and kickoffs last season.
The offensive line has just one returning starter in center Matt Stankiewitch.
Defensively, the core players that remain are led by senior linebacker Michael Mauti, perhaps the team's most vocal and outspoken leader. Mauti and end Pete Massaro, a Marple Newtown High graduate, are coming back from torn anterior cruciate ligaments.
With 29 preseason practices allowed by the NCAA, O'Brien will schedule the first week of training camp around the close of Penn State's second summer session and final exams. The first two-a-day practice will take place on Saturday.
Players may still transfer up until the Sept. 1 season opener against Ohio.
O'Brien received more bad news Sunday when wide receiver William Fuller of Roman Catholic High switched his commitment from Penn State to Notre Dame, according to multiple recruiting websites. The 6-1, 170-pound Fuller was one of six high school stars who reaffirmed their commitment to Penn State on July 28. . . . Rutgers coach Kyle Flood made it official Sunday that freshman defensive tackle Jamil Pollard of West Deptford High has joined the Scarlet Knights. Pollard transferred from Penn State.
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