Last month Shields posted a video on Instagram of him running through drills in Sarasota, Fla. In the video, he backpedaled and accelerated in quick bursts and wrote "I missed a whole year" as the caption.
He also expressed an interest in returning to the NFL even if it was not with the Packers.
"Oh yeah, most definitely man," Shields told the Milwaukee Journal-Sentinel last month when asked if he believed other teams might be interested in him. "I think they will because I can still run. I've still got that in me. I think somebody will want that."
Shields has suffered five documented concussions and was most recently concussed when his head collided with running back T.J. Yeldon's shoulder in a game against the Jacksonville Jaguars. He also missed four games during the 2015 season with a concussion.
The Packers eventually placed Shields on injured reserve, released him on Feb. 8, 2017, and he sat out last season.
Shields, who turned 30 in December, joined the Packers in 2010 and went undrafted out of Miami, where he switched from wide receiver to cornerback in his senior season.
In 80 regular-season games (62 starts), Shields has 18 interceptions for 180 yards and 245 tackles. He also recorded five interceptions in 11 postseason games for the Packers.