De Rossi has been strongly linked with a move to Manchester City, with the Italian yet to sign a new deal at the club.
City boss Roberto Mancini is desperate to land the 28-year-old as a replacement for Yaya Toure, who is on international duty at the African Cup of Nations. However, Pjanic has warned Roma directors that the team could lose its way without the influential midfielder.
"I don't even want to think about it," said the Bosnian star.
"He is a player that is so strong, I hope that he stays.
"It would be hard to lose a player that is so influential, the team would remain strong, if weakened, but the coach and the directors will do their best to keep him."
De Rossi has spent his career at Roma since joining the club in 2000.