SANTA CLARA -- Running back Brandon Jacobs, who spent his first seven NFL seasons with the New York Giants, has not been a factor with the 49ers through five games.Coach Jim Harbaugh on Monday said that Jacobs was "still not quite there" -- a reference to a knee injury that Jacobs sustained in an exhibition game Aug. 18. Jacobs has not played in a game since.But Jacobs told USA Today on Wednesday that his health is not the reason he has yet to make his 49ers debut."I feel great. I feel phenomenal. My leg is as fresh as yours," Jacobs told USA Today.Jacobs said he was ready to play last week against the Buffalo Bills and the previous game against the New York Jets. With his former team coming to Candlestick Park on Sunday, Jacobs said he would be very disappointed if he's not in uniform against the Giants."It would disappoint me a lot," he said. "But like I said, it's not my call."On Wednesday, Harbaugh said his assessment that Jacobs was not ready for game action was based on what he saw from him last week during practices."Now we're starting a new week," Harbaugh said. "So we'll determine that this week."Harbaugh did not indicate whether there were plans to activate Jacobs, who would be the 49ers' No. 3 running back behind Frank Gore and Kendall Hunter. Jacobs does not play special teams.The 49ers signed Jacobs to a one-year, 1.575 million contract after the Giants released him in the offseason. He confirmed to USA Today that he is frustrated."Oh, very," he said. "Because I don't know anything. I don't know what's what. But I'm hanging in there, I'm working every day, doing what I have to do. Let's say I'm just working and doing what I have to do and that's that."I've learned over the years when you open your mouth and say certain things, it hurts you, so I'm just going to shut up and keep working."