The Giants have the best rushing offense in the NFL, and that's not entirely due to Jacobs. The offensive line is stellar, and the duo of Derrick Ward and Ahmad Bradshaw will be more than adequate in attempting to replace the production of the studly Jacobs.
Jacobs will be fine moving forward, and probably could have played this week. The Giants just figured to play it safe and make sure he's completely healed before bringing him back. The 9-1 record and the abilities of Ward and Bradshaw had to have made the decision much easier.
I'd definitely get Ward active in almost all formats, and Bradshaw is a quality flex play or RB3 in larger leagues. I still expect the Giants to lean heavily on the run, but this does bump up the value of Eli Manning a hair.