Keasey is out for the season after he sustained a torn biceps tendon in his right arm while blocking on a kickoff return in Sunday's game against the Eagles. Keasey underwent season-ending surgery on Tuesday.

The 49ers have, in essence, gone with a 52-man roster for several weeks. Cornerback Shawntae Spencer sustained a season-ending torn ligament in his knee on Sept. 14. But the 49ers have kept him on the roster for four weeks because they have enough defensive backs on their roster and there is not anyone they could sign who would make it onto the 45-man game day roster, coach Mike Nolan said.

PLAYER NOTES

--OL Kirk Barton was claimed off waivers on Friday after the Dolphins released him on Thursday. Barton, a rookie, was a seventh-round pick of the Bears this year. Barton is scheduled to join the 49ers at the team hotel in New Jersey on Friday.

--CB Shawntae Spencer was placed on injured reserve after sustaining a torn ACL on Sept. 14 against the Seahawks. The 49ers finally placed Spencer on injured reserve to make room on the roster for offensive lineman Kirk Barton.

--RB Michael Robinson might see more action in the backfield with starting fullback Zak Keasey out for the season after undergoing surgery to repair a torn biceps tendon. Robinson, who played quarterback at Penn State, took one snap in the shotgun formation last week against the Eagles, but the play was whistled dead due to a false-start penalty.

--LB Patrick Willis leads the 49ers with 76 tackles, according to the coaches' film review. Willis also leads the 49ers with six passes defensed.

--WR Isaac Bruce moved up to a tie for ninth place with Don Maynard for career receiving touchdowns with 88. He is third in all-time receiving yards with 14,412. Tim Brown is second on the list with 14,934. This season, Bruce has 16 receptions for 303 yards and four touchdowns.

--KR Allen Rossum has returned seven kickoffs for more than 30 yards this season, as he ranks third in the league with a 26.3 average. Rossum has showed his versatility, too, as offensive coordinator Mike Martz has incorporated him into the offense. Rossum has a rushing touchdown, and he caught his first NFL pass last week for a 4-yard gain.

--DE Justin Smith has three sacks, already exceeding his total of two sacks from all of last season with the Bengals. Smith also leads the 49ers with 21 quarterback pressures, according to the team's coaches' review of game film.

--FB David Kirtman was signed off the Chargers practice squad to fill the roster spot vacated by Zak Keasey, who sustained a season-ending torn biceps tendon. There is no timetable on when the 49ers might put Kirtman in uniform to be available for games.

GAME PLAN: The 49ers did a nice job through three quarters last week of slowing down the Philadelphia Eagles' pass rush. The 49ers must find a mix of run and pass against the Giants if they have any hopes of being competitive into the second half of Sunday's game. Running back Frank Gore is the 49ers' best offensive player. Offensive coordinator Mike Martz does not need any reminding of that fact. However, Martz still shows a inclination to abandon the run at key points in games. Martz prefers an attacking mentality. When teams expect the 49ers to rely on their run game, that's when he likes to turn loose quarterback J.T. O'Sullivan. Martz might be tempted to try to beat the Giants through the air after the defending Super Bowl champions surrendered 310 yards passing to Derek Anderson of the Cleveland Browns on Monday night. Defensively, the 49ers have been very disappointing. Although they have a veteran defensive unit, the 49ers look tentative and unsure of themselves. Last week, they tried to get back to basics by just lining up and playing their base defense on first and second downs. But they still did not have much success stopping an Eagles team playing without four offensive starters. The 49ers need to get better production out of their defensive line, especially Aubrayo Franklin and Isaac Sopoaga. The secondary has not played well, but they haven't gotten a lot of help from the pass rush, either. It will be particularly important that they slow down running backs Brandon Jacobs and Derrick Ward.

MATCHUPS TO WATCH: 49ers RB Frank Gore vs. Giants run defense - Gore leads league in yards from scrimmage. He'll face a Giants run defense which had been strong before allowing 144 yards rushing on 30 carries to the Browns on Monday.

49ers LG Adam Snyder vs. Giants DL Justin Tuck: Snyder will face the bigger defensive tackles on first and second downs, but will typically confront the much quicker Tuck in passing situations.

49ers LB Patrick Willis vs. Giants RB Brandon Jacobs: Willis, the 49ers' best defensive player, will try to slow down the physical Jacobs, whom the Giants believe can have a big day running the ball up the middle.

49ers CB Walt Harris vs. Giants WR Plaxico Burress: Harris seems to fare better against big receivers, and at 6-foot-5 and 232 pounds, Burress is one of the NFL's biggest. Burress returned to action Monday night with four catches for 58 yards and a touchdown.

49ers WR Isaac Bruce vs. Giants CB Aaron Ross: Bruce has averaged 18.9 yards on his 16 receptions. Ross is 10 years younger than the man he'll be covering.

INJURY IMPACT: FB Zak Keasey is out for the season with a torn biceps tendon. He started four games this season.

--LB Manny Lawson returned to limited practice this week, and coach Mike Nolan said he is probable for Sunday's game against the Giants. If Lawson is healthy, he is expected to see more playing time. Up until his injury, Lawson was relegated to playing almost entirely on first downs.

--T Jonas Jennings did not practice this week with a shoulder injury. He has not played since the 49ers' Sept. 14 game against the Seahawks. He will not be available for action Sunday against the Giants, either.

--WR Bryant Johnson continues to be hobbled with groin and hamstring strains. He saw limited action last week against the Eagles and might not be at full strength this week, either.

--S Mark Roman (knee) did not practice early in the week, but the club believes he'll be ready for Sunday's game.

--CB Donald Strickland (hip) went through limited practice early in the week, but should be ready to play on Sunday.