The series starred Michael Chiklis as corrupt cop Vic Mackey and ran for seven seasons between 2002 and 2008.
In a recent interview, Ryan told the New York Times: "Obviously [Michael and I are] both doing other stuff at the moment. I wouldn't say that I'm overly pessimistic, but some things have to fall in place for it to happen."
"We've all seen how difficult it's been to launch an Arrested Development movie and a 24 movie," he continued. "These things take a little time and a little finessing."
The Chicago Code creator also cited the recent departure of Fox executive Alex Young as a reason for the project's long gestation.
"[He] was really behind the idea [and] has since left," he said. "So we'd have to start over in terms of pitching it, and I don't think Michael or I have had the time.
"I certainly don't want to say it's never going to happen, but I also don't want to give fans false hope that it's right around the corner."
During its run, The Shield won a Golden Globe for 'Best Drama Series' and Michael Chiklis was awarded both a Golden Globe and an Emmy for his central performance.