After leaving Yamaha for Ducati in 2011, Rossi endured his first ever win-less season at world championship level.
But Lorenzo, who was Rossi's Yamaha team-mate from 2008-10, dismissed any suggestion that the Italian seven-time MotoGP champion's career was in decline.
"There's no age limit," Lorenzo told Gazzetta dello Sport. "[Carlos] Checa at 39 in superbikes is in his top form. Valentino is 33, he hasn't weakened."
Lorenzo suggested that Rossi's problem was not that his talent was fading, but that MotoGP was now so competitive it was harder to pull off good results on a difficult bike than it had been in the past.
"Today in MotoGP there are many strong riders," Lorenzo said. "To win you need a great feeling with your bike."
The Spaniard thinks Ducati faces a tough task to close the gap on Honda and Yamaha in 2012, but sees no reason why it cannot eventually rediscover title-winning form.
"It's an unknown quantity," said Lorenzo of Ducati. "It won't be easy to reach Yamaha and Honda, but Ducati beat them in the past. It's a small factory but with a lot of technology and a great rider in Valentino."