making San Francisco's starting defense whole for the first time this season.
He'll be welcomed back with open arms.
"Julian brings a different aspect," said linebacker Derek Smith, who leads the Niners with 26 tackles after two games - more than double any other player on the team. "He has a lot of speed, he's great rushing. The speed, smoothness and athletic ability he brings kind of makes it for you can have some different packages, as far as blitzing and stuff like that."
The Niners can use the kind of spark Peterson can provide. After opening the season with a strong performance Sept. 9 against Atlanta, the Niners plummeted from fifth to 20th in the NFL defensive rankings last week after allowing 424 yards to the St. Louis Rams in a 30-26 defeat.
Peterson, who missed San Francisco's first two games with an ankle injury, is expected to start Monday night against the New York Jets. The team's first-round draft pick last year, the rangy Peterson is adept at covering tight ends, playing the run and, especially, rushing the passer. His versatility gives San Francisco a dimension it has been missing since Peterson was injured in the preseason finale.
"Really, this is the first time the entire defense has been together this year," Niners coach Steve Mariucci said Wednesday. "We weren't ever all together in a preseason game. We always had a few guys out."
Peterson is regarded as one of San Francisco's top impact defenders. Despite missing three games to injury as a rookie, he was third on the team with four sacks and third with two interceptions.
"I'm excited to be back," Peterson said. "I'm about 85 percent, but I'm good enough to perform. Hopefully, I can create some positives. I'm focused and ready to go out there and perform for my team."
The Niners will be looking for Peterson to add a little more performance to their pass rush. St. Louis quarterback Kurt Warner passed for 224 yards and two touchdowns in the second half last week as the Niners were unable to reach him for a sack after halftime. The Niners have just three sacks through two games.
"I'm definitely eager to bring a little more heat, any kind of added pressure I can bring," Peterson said. "That's about all I can see we need. As far as coverages and everything else, the rest of the linebackers and defense as a whole is doing great. It's good to be back out there, and now I'm just trying to make something happen."