Bruce has been working closely with the Rams during the off-season, sharing his knowledge of the game with the inexperienced receivers on the roster, none of whom have played more than two full seasons in the NFL.
The 40-year-old believes that Pettis has had a change of mentality during the close season, which could help him become a dominant player for the Rams.
"He is a starter right now," Bruce told reporters. "He's not a guy who is just kind of glad to be here anymore. From what I see, I think he's got the mentality of a number one receiver and that's big.
"If you are going to be a dominant guy in this league, I think that's the right mentality to have."
Bruce won one Super Bowl and broke numerous receiver records during his 13 years with the St Louis Rams.