SANTA CLARA Perhaps the 49ers' most costly play of the year came on an innocuous-looking two-yard run at the end of the third quarter Sunday.
Running back Kendall Hunter and wide receiver Kyle Williams were injured on the play, and both will miss the rest of the season after being placed on injured reserve Tuesday.
Hunter, the 49ers' second-leading rusher, tore his Achilles' tendon; Williams tore the anterior cruciate ligament in his knee.
"That's a big loss," running back Frank Gore said of Hunter. "I respect his game. Everyone on the team respects his game. Kendall can make a big play any time in the game."
Another running back, Anthony Dixon, said losing Hunter for the season was like "losing a brother."
Hunter has rushed for 371 yards this season, scored two touchdowns and highlighting his big-play ability has averaged 5.2 yards a carry. He also has lightened the load for Gore, who has had a few minor injuries but has started every game.
The 49ers have plenty of options to fill Hunter's role.
Veteran Brandon Jacobs won two Super Bowl rings with the Giants but only has rushed once for one yard this season. Dixon also could take on a bigger role; he has 12 carries for 48 yards.
Second-round draft pick LaMichael James also is available, but he hasn't even been in uniform this season. He's the most similar to Hunter in terms of speed and stature, and he also developed a strong rapport during the offseason with Colin Kaepernick, the team's starting quarterback the last two games.
Both played in spread-like offenses in college, and they were successful on the read-option play during the preseason. James even lived at Kaepernick's Santa Clara apartment during the spring and summer.
"He's been practicing hard," Gore said when asked about James. "I know he wants to play. I know if coach (Jim) Harbaugh decides that he's the next guy (in at running back), coach (Tom) Rathman will get him ready to play."
The 49ers have one more option.
With two open roster spots, they activated rookie Jewel Hampton, who had been on the non-football injury list since having surgery to remove bone spurs in his ankle in June.
Hampton (5-foot-9, 210 pounds) scored seven touchdowns as a freshman at Iowa. He later transferred to Southern Illinois and rushed for 1,121 yards and 17 touchdowns last season.
There are only 52 active players on the 49ers' 53-man roster, meaning they could make another move this week.
Et cetera Kaepernick and Alex Smith were not made available to reporters Tuesday. Harbaugh is expected to announce today who will start Sunday's game in St. Louis.
The 49ers signed former Eagles receiver Chad Hall to the practice squad and released safety Eddie Whitley from the squad. Hall played 15 games for the Philadelphia Eagles during the 2010 and 2011 seasons as a receiver and runner, and he returned punts and kicks.
The 49ers are trying out kickers this week. David Akers has struggled, including Sunday when he missed a 50-yard attempt and had a 33-yard attempt blocked. The team does not release the names of tryout players.