Steve Bartkowski a doubtful starter after injuring his shoulder Monday night, scored on a quarterback sneak and threw a 37-yard touchdown pass yesterday to lead the Atlanta Falcons to a 21-10 victory over San Francisco, ruining the 49ers' coaching debut of Fred O'Connor.

O.J. Simpson, second leading rusher in National Football League history, separated a shoulder during the second quarter and will be out for an indefinite period.

"I am in a little pain," Simpson said, his arm in a sling after his postgame shower.

"When I went down, I heard something snap," he said. "The doctors told me it was a third degree separation. I'm not sure what that means but they did say it's worse than a first degree."

Simpson said he was injured when Atlanta's Jeff Yeates and Wilson Faumuina fell on him on the tackle.

O'Connor said doctors examined Simpson at halftime and the diagnosis at that time was that his right shoulder was separated. X-rays later confirmed the separation.

Simpson left the game after a two yard gain about five minutes into the second period. He had rushed four times for 13 yards.

"We hate to lose O.J.," said O'Connor. "We don't know how long he will be out. It will be a problem for us. It will certainly hurt us."

Bartkowski, who stretched a shoulder ligament in Atlant's upset of Los Angeles, went across from one foot out on the opening play of the second quarter to give the Falcons their first lead in the game. He threw the scoring pass to Wallace Francis to cap an 84-yard drive with 3:24 left in the third period.

The first touchdown was set up when Larry Jones bobbled a punt and Atlanta linebacker Dewey McClain recovered the ball at the 49er 27-yard line.

The Falcons, who have won four in a row to improve their record to 6-4, got their other touchdown midway through the final period on a one-yard run by Bubba Bean after Frank Reed had returned a pass interception 38 yards to the San Francisco three-yard line.

O'Connor, an assistant to the ousted 49er coach, Bill McCauley, was promoted to head coach Tuesday.