Waters has not played in the NFL since turning out for the New England Patriots in their ultimately unsuccessful 2011 Super Bowl appearance.
Garrett acknowledged that Waters had had a fruitful career in the league, but expressed concerns over the 36-year-old's lack of match practice.
"Brian Waters has been a really good player in this league for a long time," Garrett said.
"As you guys know, I think he's a six-time Pro Bowler. He's an NFL Man of the Year, played a long time in Kansas City a few years ago, he came late to New England and had a Pro Bowl year for them when they went to the Super Bowl.
"The biggest issue for Brian right now is he hasn't played in 18 months. You have to factor those things in."
Waters is most notable for his time with the Kansas City Chiefs, where he spent 10 years between 2000 and 2010.