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The makeup of the San Francisco 49ers has changed a lot since last season, and that helps explain why they’re now in an uphill battle to earn a playoff spot.

If running back Garrison Hearst had not broken his left fibula on the 49ers’ first offensive play in the divisional playoff game last season against the Atlanta Falcons, they might have advanced to the NFC championship game. Instead, they lost 20-18 to the conference’s representative in the Super Bowl.

So why the drastic fall in such a short period? The answer is simple. The 49ers simply do not have the personnel they had last season. Here’s a breakdown of what happened at several important positions.

Last season: Steve Young.

This season: Jeff Garcia.

Why: Quarterback Young sustained a possible season-ending concussion Sept. 27. General Manager Bill Walsh traded backup Ty Detmer to Cleveland for a fourth-round pick one week after signing Garcia as a free agent. The move cost the 49ers about $100,000 in salary-cap space.

The trade-off: Young was coming off one of his best seasons. He struggled in the first two games, but his lowest passer rating since he became the starter was 92.3 in 1995, when he missed five games because of injuries. Garcia played five seasons in the Canadian Football League and is auditioning for the full-time job after Young is gone. His play has been erratic.

Last season: Garrison Hearst.

This season: Charlie Garner.

Why: Running back Hearst underwent surgery in July to correct a circulatory condition in his left ankle resulting from the severe sprain he incurred in a playoff game last January. He probably will not play this season. Garner and Lawrence Phillips were signed just before camp.

The trade-off: Hearst rushed for a team-record 1,570 yards last season and blocked impressively. Garner has performed well and given the team its only big-play threat. He is on pace to rush for 1,000 yards. Phillips has not been much of a factor with his difficulty learning the offense.

Last season: Marc Edwards.

This season: Fred Beasley.

Why: Fullback Edwards was traded to Cleveland for a fourth-round pick on a day the 49ers also announced the signing of Tommy Vardell to take his place. The move did not save the team any cap dollars.

The trade-off: Edwards did everything asked of him last season as a blocker, part-time runner and part-time receiver. Vardell lost his starting job to Beasley after four games. Edwards and Beasley are being used primarily as blockers this season. Edwards has just 6 yards in two carries and 13 receptions for 130 yards and a TD. Beasley has 25 yards in 10 carries with eight receptions for 86 yards.

Last season: Kirk Scrafford.

This season: Derrick Deese.

Why: Scrafford, a tackle, retired at the end of last season because of a chronic neck condition. That decision saved the 49ers $6 million this season in their attempt to cut $28 million and get under the salary cap. Deese, who played guard on the 1994 Super Bowl team, split time last season at both tackle positions.

The trade-off: Scrafford came out of retirement midway through last season and within weeks asserted himself as the best lineman on the team. Deese is good in pass protection but not as stout in the running game.

Last season: Kevin Gogan.

This season: Jeremy Newberry.

Why: Lineman Gogan, whose style clashed with the preference of offensive line coach Bobb McKittrick, was traded to Miami for a fifth-round pick. Newberry was switched from right tackle after the second game this season.

The trade-off: Gogan made the Pro Bowl for the second straight season in ’98 and was a big reason Hearst had so much success running between the tackles. Also, the pocket rarely collapsed in the quarterback’s face with him around. Newberry, a second-year player, appears better suited to guard after switching positions with Deese.

Last season: Chris Doleman.

This season: Gabe Wilkins.

Why: Doleman, who led the team with 15 sacks, retired after last season. When he wanted to come back for another season, Walsh had no interest because he already had brought Charles Haley out of retirement with a $1.2 million signing bonus. Wilkins plays in base situations.

The trade-off: Doleman is rounding into shape with the Minnesota Vikings. Wilkins, who has no sacks, is fairly sturdy but unspectacular against the run. Haley replaces Wilkins on pass-rush downs and has two sacks and seven tackles.