"I think we dodged a bullet there," Mariucci said. "I'm really pleased to hear it's just a matter of weeks, because when I was driving home last night, I was worried it might be season-ending. We're lucky because it looked like he got hurt pretty bad."
Ulbrich has bruising and swelling in the knee, but he won't require surgery and could be back as soon as the Sept. 15 game against Washington if his rehabilitation goes well.
Ulbrich had solidified his standing as the starting right inside ("plugger") linebacker during training camp and the preseason, holding off a strong challenge from second-year player Jamie Winborn, who now will take Ulbrich's place in the lineup.
"We will plug in Jamie Winborn as the plugger and we will hit it running," Mariucci said. "Jamie started some games last year, he's very capable and he'll do a good job. He's very valuable to us. But we need Jeff Ulbrich back - no question about that - and he will be back."
The Niners already are banged up at linebacker, where impressive rookie Saleem Rasheed is nursing a quad bruise that could keep him out of Wednesday's preseason finale against San Diego.
Those injuries improve the chances to make the team of Quincy Stewart, Frank Strong and Brandon Moore, who all have performed well in camp and the preseason and have impressed coaches with their play.