[Update: Following a poor workout, the Cowboys passed on Jackson after all, Josh Ellis of dallascowboys.com now reports. It's unclear whether the Cowboys plan to bring in anyone instead.]
The Cowboys are expected to release Bill Nagy to make room for Jamaal Jackson on the roster. If Nagy is not claimed off waivers by another team, which is highly unlikely, the Cowboys are expected to put him on IR for the rest of the season. According to Josh Ellis, Nagy will require "serious surgery" to fix what had initially been described as a high ankle sprain.
As had been the case with the previous free agent linemen brought in, Jackson will have to wait three days after signing his contract before he can practice with the Cowboys. The move fills a short-term gap for the Cowboys, but doesn't rule out further free agent activity later on to bolster the center position.