SAN FRANCISCO — Shortly after the 49ers welcomed Michael Crabtree back for his season debut, they saw left tackle Joe Staley exit Sunday's game with a right-knee injury on that opening series.

Staley got hurt blocking on a Frank Gore run midway through the 49ers' first possession, which ultimately ended with a 32-yard field goal from Phil Dawson for a 3-0 lead over the St. Louis Rams.

As Staley walked with the team's medical staff to the locker room, he promptly was ruled out from returning. Staley has started every game since 2011. Alex Boone replaced Staley, and Joe Looney entered at right guard in his NFL debut.

Earlier, Crabtree started in his season debut, just over six months since tearing his right Achilles tendon. Last season's leading wideout en route to the Super Bowl, Crabtree replaced Mario Manningham in the lineup to start opposite fellow wideout Anquan Boldin.

Crabtree drew two penalties on the 49ers' opening series: an illegal block in the back that negated a 14-yard run by Frank Gore, and offensive pass interference on a ball he failed to catch.

Crabtree was in for all but one play on that opening drive, which stalled near the Rams' 15-yard line.

The 49ers averaged a league-worst 173.5 passing yards per game entering Sunday's visit by the St. Louis Rams.

Crabtree was the 49ers' first-round draft pick in 2009. He is coming off a career-best season, totaling had 85 receptions for 1,105 yards and nine touchdown in the regular season, then 20 catches for 285 yards and three touchdowns in the postseason.

Rounding out the 49ers' wideout corps Sunday were Boldin, Manningham and Kassim Osgood. Deactivated were Jon Baldwin (fifth time in 11 games) and Quinton Patton (eighth straight game because of a foot fracture).

In other lineup changes, outside linebacker Aldon Smith started his first game since Week 3, and he promptly made a tackle on the opening snap.

Defensive tackle Ray McDonald is suiting up after a two-game absence, a result of a sprained left ankle.McDonald got credited with a sack on the second snap, which was set up by Justin Smith's pressure.

Left guard Mike Iupati (left knee) and cornerback Tarell Brown (ribs) missed their second straight games.

For more on the 49ers, see Cam Inman's Hot Read blog at blogs.mercurynews.com/49ers. Follow him on Twitter at twitter.com/CamInman.