It was not specified what banned substance was at issue with Shaw, but coach Dennis Erickson said he will enter a four-week rehabilitation program in Florida and referred to Shaw's suspension as a "drug violation."
Erickson also said that Shaw entering the program was not voluntary, but "mandated."
Shaw will remain on the team's roster during the suspension but will not count against the 53-man limit the Niners will carry into the season.
Shaw is eligible to return to the 49ers' active roster on September 29 following the team's game against Minnesota the day before. The Niners then will have an unspecified window, perhaps as long as a week, to decide either to waive Shaw or keep him on their roster.
"He comes back, then we address the situation with him," Erickson said. "If he comes back, goes through waivers, (we could) put him on the practice squad."
That comment seems to indicate what the Niners had planned for Shaw if he hadn't been suspended by the league.
The team had high hopes for the Michigan State
product and general manager Terry Donahue said he could become a draft-day steal when the Niners selected him in the fifth round in 2002.
A serious knee injury suffered midway through his senior season dropped Shaw's stock, and he spent most of the season rehabbing the knee before appearing in a few games at the end of last season, when he recorded three tackles and one sack.
Shaw spent most of the past spring continuing to strengthen the knee and rehabbing a left shoulder injury on which he had offseason surgery. He came to training camp expecting to challenge for a spot in the regular rotation at defensive tackle.
But Shaw, who was bothered by an ankle injury that kept him out of Saturday's preseason game against New Orleans and also forced him to miss some training camp practice, had been falling out of the team's plans due to unimpressive performance in camp and the preseason. It was unlikely he was going to earn a berth on the final 53-man roster.
When asked if Shaw had been seriously challenging for a roster berth before the suspension, Erickson said, "I think at one time he was. ... He could have at one time, but as it turned out, we don't know who are fifth guy (at defensive tackle) is going to be."
The Niners, in fact, are unlikely to keep more than four defensive tackles on their final roster and appear set to go into the season with Bryant Young
, Jim Flanigan
, Anthony Adams
and Travis Kirschke
at that position.
The Niners appear likely to waive Shaw when his suspension is over. Depending on what their situation at defensive tackle is at that time, they may consider signing him back to their practice squad, for which Shaw still is eligible.
But Erickson said that's an issue for later, and not what is important regarding Shaw right now.
"Obviously, he has to get himself straight," Erickson said. "The biggest thing is he takes care of himself, gets himself straightened out however he has to do that. That's the most important thing. He can worry about the other things after that."
The NFL office made the decision on Josh Shaw for the 49ers on Tuesday. Shaw, the second-year defensive tackle who was on the roster bubble, was suspended without pay for the team's first four regular-season games for violating the league's substance abuse policy. The Niners must address Shaw's roster status again at the end of September, but it now appears unlikely he will play again for the team.