NAPA, Calif. -- On Thursday, the Oakland Raiders officially notified the NFL that halfback Napoleon Kaufman retired to devote his life to the ministry.

	Wisniewski

On Friday, Raiders coach Jon Gruden said that six-time Pro Bowl guard Steve Wisniewski didn't report to training camp because he might be contemplating a similar move.

Wisniewski, 34, spent Friday at an undisclosed seminary in Southern California, thinking about whether he wants to continue playing. Like Kaufman, Wisniewski was one of the most religious players on the team. The two are close friends.

"I think every year at this time, I'm sure he's contemplated whether he wants to continue playing," Gruden said. "I have not heard that he doesn't, but I do know he's in Southern California right now. I hope to see him soon."

Gruden declined to speculate any further about Wisniewski's surprising absence. The veteran guard was one of four players who didn't arrive for the opening of training camp. Also missing was receiver-running back Eric Metcalf, who spent an extra day in Seattle cleaning up personal matters, and two unsigned draft choices -- first-rounder Derrick Gibson and third-rounder DeLawrence Grant. The team hoped to get an agreement with Grant before the end of the weekend.

Losing Wisniewski could be a huge blow for the Raiders' hopes to advance past the AFC championship game and reach the Super Bowl. Wisniewski has not only been a leader on the team for the past 12 seasons, but he's been one of the best players on the team.

"He's a real deep thinker," Gruden said.

Gruden seems to be optimistic that Wisniewski would eventually return to the team. For the moment, he doesn't plan to make any adjustments on the offensive line in Wisniewski's absence. Frank Middleton, a former starter for the Tampa Bay Buccaneers, will work behind Mo Collins at right guard. Neither will be moved to Wisniewksi's left guard position. Tony Hutson is listed as Wisniewski's backup.

Gruden said he is not ruling out the return of wide receiver Andre Rison, who faces a four-game suspension for not fulfilling all the terms of the substance abuse policy.

John Clayton is a senior NFL writer for ESPN.com.