Under NFL rules governing short-term injured reserve, Jackson now has to miss at least eight games. He's eligible to start practicing again in six weeks. According to a source, it's regarded as a six-week injury. Jackson is in a protecting walking boot.
"I'll be back," said Jackson, who has 19 tackles in four starts this season. "It could have been much worse, so that's the good thing."
Veteran Lardarius Webb will take his place in the starting lineup.
Franks was cut by the Ravens during the final major roster cutdown after recording eight tackles, two pass deflections and one interception during the preseason.
"Dominique is going to do a good job for us," Harbaugh said. "He's a good football player. We liked him then, and we were fortunate that he's still available."
Franks was on his way to try out for the Detroit Lions on Tuesday when the Ravens called Monday. Franks hasn't been with a team since being cut by the Ravens, trying out for the Tennessee Titans in September.
"Man, it's great to be back," said Franks, a 2010 Atlanta Falcons fifth-round draft pick from Oklahoma who has recorded 30 career tackles and three interceptions. "I was sad I had to leave in the first place. The defense hasn't changed any, so it makes me more comfortable to get back in the swing of things. I've got to prove myself in practice. Hopefully, my number is called."