A source told ESPN's Ed Werder that Robbins has checked into a hospital in San Diego, where he has undergone tests that show no illegal drugs in his system. The source said that further tests will be conducted.

When contacted for confirmation by ESPN, Robbins' agent, Drew Pittman, said: "My comment is that I have no comment."

In December of 1996, Robbins was hospitalized with a "chemical imbalance complicated by influenza syndrome," then-team doctor Robert Albo told the San Francisco Chronicle. At the time, Robbins was on prescription medication to address a sleeping problem, according to the same '96 Chronicle report.

The Scripps spokeswoman, Marg Stark, said there was a medical and emergency plan in place by the NFL to treat players who needed medical attention but did not require hospitalization. These players were to be treated by NFL medical personnel. She said that most likely Robbins was receiving medical treatment under that previously arranged procedure.

Robbins was not with the Raiders for their walk-through Saturday at Qualcomm Stadium. The Raiders' first contact Saturday with Robbins, whose whereabouts that day are not known, was at 8 p.m. PT, shortly before their 8:45 final team meeting. Robbins was told not to attend the meeting.

All coach Bill Callahan told the team during the Saturday night meeting was that Adam Treu would start at center against Tampa Bay and added to his players that "no one is bigger than the team."

Callahan gave no further details to his team.

Raiders owner Al Davis declined comment, saying, "Let's play the game.''

Robbins, who has been bothered by a foot injury for much of the season, had been questionable against the Bucs.

Raiders linebacker Bill Romanowski, interviewed by ABC's Melissa Stark on Sunday, was asked whether the change at center was a distraction to the team. "It really isn't," he said, declining further comment. Raiders players were just finding out this morning that Robbins had been sent home and wouldn't play. The Raiders lost the game 48-21.

When asked about Robbins missing the game, Buccaneers linebacker Derrick Brooks said: "We don't prepare for individuals. We prepare for a team. We don't take backup players lightly. Their job is to come in and play as well as the starter. And we expect that."

Robbins, a 6-foot-3, 320-pound player in his eighth season, was expected to help contain Tampa Bay's formidable defensive front, led by All-Pro tackle Warren Sapp. Robbins anchors a line that gave league MVP Rich Gannon enough time to throw for 4,689 yards and direct the NFL's most potent offense.

Robbins was selected to his first Pro Bowl this season, joining tackle Lincoln Kennedy on the team.

Robbins broke into the Raiders' starting lineup in 1996. He injured his right knee last season and missed the final 14 games. The 6-foot-5, 300-pound Treu took his place.

At the 1989 Super Bowl, fullback Stanley Wilson of the Cincinnati Bengals was found in his Miami hotel room in a cocaine-induced stupor the night before the game. He did not play, and Cincinnati lost to San Francisco, 20-16.

Ten years later, safety Eugene Robinson of Atlanta was arrested less than 24 hours before the Super Bowl for solicitation of prostitution. He played, and was beaten on a touchdown pass by John Elway as the Falcons lost to the Broncos, 34-19.

Information from The Associated Press was used in this report.