It was a matchup of two old neighbors who have combined to win four of the last five Super Bowls. But only the San Francisco 49ers looked like a possible champion today. The Raiders seemed to be a team lost between Oakland and Los Angeles, or at least between the 20s.

The 49ers defense allowed the Raiders inside the 20-yard line only once and defeated Los Angeles, 34-10, in front of a sellout crowd of 87,006 at the Los Angeles Coliseum.

To make matters worse for the Raiders (1-2), quarterback Jim Plunkett suffered a separated left shoulder when Jeff Stover sacked him with about 10 minutes left in the game. The Raiders said Plunkett, who is right-handed, would be out for at least six weeks. He was to be hospitalized tonight for observation, but it was not known if he would require surgery.

As he has done in the past, Marc Wilson will step in for Plunkett at quarterback.

Perhaps the change will inject some life into the Raiders offense. "It's only the fourth week of the season, not the 14th, but it's do or die time time for us (next week at New England)," said Los Angeles cornerback Lester Hayes. "Our offense has to take some pressure off our defense."

Today's game was relatively close until a San Francisco surge late in the third quarter. The 49ers (2-1) took a 20-3 lead with 2:27 left in the period when Joe Montana threw a 14-yard touchdown pass to Dwight Clark. On the first play after the kickoff, Plunkett fumbled as he was being sacked.

San Francisco linebacker Milt McColl picked up the ball at the 28 and ran in for the score that made it 27-3.

As the extra-point team took the field following McColl's touchdown, satisfied San Francisco Coach Bill Walsh removed his headphones and smiled for perhaps the first time all day.

Raiders Coach Tom Flores, whose team has been outscored by 40 points in the last two weeks, had no reason to smile because the 49ers defense was so dominant.

"The 49ers' defense definitely means as much to them as their offense," he said. "There's no question about that. They have an excellent defense."

The Raiders didn't get a touchdown until Wilson scored from the one midway through the fourth quarter.

After Los Angeles received the opening kickoff and punted, the 49ers got the ball on the Raiders 48. Four plays later, the 49ers had a touchdown, running back Roger Craig catching a 20-yard touchdown pass from Montana.

After San Francisco's Ray Wersching and Los Angeles' Chris Bahr traded field goals, San Francisco scored right before halftime. On third-and-10 from the Raiders 48, Jerry Rice streaked by Hayes for a 39-yard catch to the nine with 73 seconds left in the half. On the half's second-to-last play, Wersching kicked his second Rice-arranged field goal, a 26-yarder.

The Raiders finally got some workable field position when Wendell Tyler fumbled on the first possession of the second half. Los Angeles recovered at the San Francisco 38.

Three plays later, it was fourth and two at the 30. The Raiders, needing a touchdown and almost out of field-goal range, went for it. Plunkett threw a short incompletion over the middle intended for Frank Hawkins.

After an exchange of punts, the 49ers mounted the drive that made it 20-3. They went 80 yards in nine plays, including another big catch by Rice, a leaping 23-yarder. The touchdown came on a third-and-three play from the 14 as Montana rolled right and threw across his body to Clark, open at the seven.

The 49ers offensive line afforded Montana plenty of time to throw most of the afternoon. Maybe Plunkett's offensive linemen should have tried something Los Angeles defensive end Howie Long was accusing 49ers blockers of doing: leg-whip blocking, an illegal tactic. Long and Bobb McKittrick, the 49ers offensive line coach, had a shouting match on that subject as the teams were going up the tunnel after the game.

Long then refused to talk to the media. That's unusual. But so is seeing the Raiders with a 1-2 record, and seeing them get outplayed on defense, and hearing them accusing someone else of illegal deeds. With the 49ers looking again today like the only football team the Bay Area ever will need, does Oakland really still want the Raiders back?