During a recent Los Angeles radio interview, Barber said Giants players are "not listening to Tom Coughlin anymore," and called for the coach's dismissal.
Asked about Barber's comments on Wednesday, Manning sarcastically said, "That's nice of him. It's good to hear from Ol' Tiki," according to NJ Advance Media.
Manning was also asked if it's a distraction when a former player is critical of a head coach. He answered that it "depends on your opinion of that player."
Coughlin has been the Giants head coach since 2004. Barber retired at the end of the 2006 season, and New York and Manning went on to win the Super Bowl the next year.
Barber has criticized Coughlin in the past, writing in a book he published in 2007, "If Tom Coughlin had not remained as head coach of the Giants, I might still be in a Giants uniform."
Manning said Wednesday that a lack of effort is not the reason for the 3-7 Giants' struggles this season.
"Guys are practicing hard, guys want to do well, guys are into it and excited about the week," he said. "They're doing all the right things in the preparation, we've just got to keep doing the right things and take it over to game day. You can't have the same mistakes that all costing us games."
Coughlin has a 93-77 overall record as New York's head coach, including two Super Bowl wins. The Giants have missed the playoffs in each of the last two seasons.
- Molly Geary