According to Sky Sports, the 29-year-old told reporters at a press conference: "Sergio is someone I know well and for me is one of the five best strikers in the world.
"Given space he is the type of player who can destroy you so we have to ensure he doesn't get that space and doesn't feel comfortable."
Barcelona will host Manchester City at the Nou Camp on Wednesday and hold a two goal advantage going into the second leg.
City defender Martin Demichelis was sent off and Lionel Messi scored the resulting spot kick in the first leg at the Etihad Stadium. Dani Alves then doubled the lead late on leaving City with a mountain to climb in the second leg.
But the ex-Liverpool man insists that Barcelona must be wary of City's attacking threat.
"I think we are going to see Man City go all out to attack," he added. "That is the way they normally play and with the result they'll need to do that more than ever."
City are hoping to bounce back after a shock 2-1 defeat by Wigan in the FA Cup on Sunday. And the English side will fancy their chances as the Catalan team are also experiencing a rare dip in form and confidence.
Last weekend the Spanish outfit lost 1-0 to Real Valladolid who were 18th in La Liga at the time.
Write for GiveMeSport! Sign-up to the GMS Writing Academy here: http://gms.to/1a2u3KU