That’s not easy to do for the San Francisco 49ers right now.

Manningham is out of the season with a left knee injury after getting tackled low by Leroy Hill and fumbling in the third quarter of San Francisco’s 42-13 loss at Seattle. The injury to Manningham, one of San Francisco’s signature additions in the offseason, is yet another blow to an increasingly thinning wide receiver corps.

Kyle Williams already is lost for the season with a left knee injury and tight end Vernon Davis did not participate in Wednesday’s practice while he recovers from a concussion. Davis took a huge hit from Seattle’s Kam Chancellor and must be cleared by an independent neurologist before he can return.

With so many injuries to key receivers, the 49ers (10-4-1) will rely more than ever on Randy Moss and first-round pick A.J. Jenkins to support top threat Michael Crabtree, starting Sunday against the Arizona Cardinals (5-10) in the regular-season finale. Niners coach Jim Harbaugh also said Ricardo Lockette or Chad Hall will likely be promoted from the practice squad.

“That group will have to step up,” Harbaugh said.

The loss of Manningham is a significant setback for a team that counts on Crabtree as its only healthy wide receiver with more than 26 receptions this season.

Crabtree has 77 receptions for 933 yards and seven touchdowns. Manningham’s over-the-shoulder catch between two defenders on the sideline in the Super Bowl propelled the New York Giants’ winning TD drive against the Patriots last February. He ranks second with 42 catches for 449 yards and a touchdown after missing some time previously with a shoulder injury.

After those two, San Francisco has received limited production from its other wide receivers.

Moss, two months shy of his 36th birthday, has 26 receptions for 406 yards and three touchdowns. Ted Ginn Jr., has two catches for 1 yard. And Jenkins, drafted 30th overall out of Illinois, has been active for three games and has not been targeted.

About the only positive for San Francisco is that second-year quarterback Colin Kaepernick often worked with the backups and practice squad players before being promoted over Alex Smith six weeks ago.

“I think that’s why we’re not too worried about it,” said Kaepernick, who is 4-2 since taking over for Smith. “I’ve had a lot of practice with those other guys.”

Just not when it counts.

Jenkins couldn’t crack the lineup at the outset with Crabtree, Manningham, Williams and Moss ahead of him on the depth chart — not to mention Davis, Delanie Walker and the other tight ends in Harbaugh’s complicated version of the West Coast offense. Frank Gore also has 25 catches for 213 yards out of the backfield.

At the most important time of the season, now it’s Jenkins and other wide receivers with little experience suddenly being thrust into a bigger role.

The 49ers can still secure a No. 2 playoff seed — and the first-round bye that comes with it — with a win against the Cardinals coupled with a Green Bay loss at Minnesota. If they lose and Seattle (10-5) wins at home against St. Louis, the Seahawks will steal the NFC West and send San Francisco on the road for the first round.

Jenkins admits this year has been one of the most frustrating of his life. He has spent most of the season watching and asking questions of the veterans, and he’s eager for the opportunity to show his worth when it matters most.

“Going through my whole career, I never really had to sit out,” Jenkins said. “This is my first time doing it. It’s different. You never really saw the game from the sideline as much as I did. But now it’s my opportunity to play, so I have to be ready for it.”

The most important role of the other receivers in line to help replace Manningham has, until this point, been to simulate the opposition on the scout team.

Lockette is playing the part of Cardinals wide receiver Larry Fitzgerald this week, a role he also did before San Francisco won 24-3 at Arizona in October. Lockette had two catches for 105 yards, including a 61-yard touchdown, with the Seahawks last season and is likely to be active ahead of Hall on Sunday because of his speed and athleticism.

Lockette said he felt helpless watching the 49ers get whipped in Seattle and Manningham go down with a season-ending injury. He has not been told if he’ll play Sunday.

“I wanted to play so bad,” Lockette said. “Sitting on the sideline, you feel like you could have done something or you could have helped out right here or there. Hopefully I’ll get my chance in the near future.”

NOTES: DL Justin Smith was not on the practice field Wednesday during the portion open to media. His streak of 185 straight starts ended when he sat out with an elbow injury at Seattle, and his status for the playoffs remains murky. … The 49ers signed DT Lamar Divens and S Curtis Taylor to the practice squad.