SAN FRANCISCO -- Brandon Jacobs can barely hide his frustration. Because of a preseason knee injury, the former Giants running back has yet to get on the field this season for his new team, the 49ers.

Last week, Jacobs maintained the knee injury was healed. Yet Jacobs was sitting on the inactive list the last two games, against the Bills and now the Giants.

This week, Jacobs did express his frustration briefly to USA Today, saying he didn't know what the 49ers coaches were doing. He backtracked after a few meetings with coach Jim Harbaugh, who doesn't take kindly to players airing dirty laundry in the media.

It's something wide receiver Braylon Edwards did last year. In a surprise to many of the 49ers players, Harbaugh cut him at the midway point.

Perhaps with that mind, Jacobs was careful when talking to reporters after his former team crushed his current team, 26-3 at Candlestick Park on Sunday.

"This is the toughest game I've ever had to watch, period," Jacobs said. "Period. I have never been able to help my team in a role like this. But I'm getting better, am doing better every week."

Jacobs then played the good soldier.

"I am practicing, I'm running hard, I'm doing everything I need to do to get myself comfortable, which is a big part of it, working until you get comfortable," he said. "I'm happy with the way they are doing it."

It seems now, the only way Jacobs gets on the field is if another running back gets hurt. Frank Gore is still the unquestioned starter and Kendall Hunter is his main backup, which leaves few carries for anyone else.

That means the backup runners who do play must play special teams, which is something Jacobs rarely if ever did as a Giant.

The 49ers really haven't needed him so far. They came in as the top rushing offense in the league. Nevertheless, they were throttled by a Giants defense that held the 49ers to only 80 yards on 17 carries.

"Brandon is a great player," former teammate Ahmad Bradshaw observed. "I don't know what their coaching staff is thinking or what they expect from Brandon. I think they are losing a lot by not using him."