The wide receiver started his career playing alongside Manning at the Indianapolis Colts, before moving to the NFC East side in 2012 for RG3's rookie season in the league.
The 26-year-old told NFL AM: "[Griffin] can be just as good as Peyton.
"Peyton's very skilled with his arm, Robert is skilled with his arm and his legs, so that's what makes him more dynamic and more dangerous.
"But they are very similar, Robert's going to be very good, and just as good as Peyton."
Griffin was named the offensive rookie of the year after leading the Redskins to the playoffs last season.