The seven-time MotoGP champion has only two podium finishes to show from his two seasons at Ducati, while his team-mate Lorenzo clinched the 2012 title last weekend at Phillip Island.
The duo were paired at Yamaha from 2008 to '10, with Rossi winning the championships in the first two of those three seasons before Lorenzo claimed the 2010 crown, albeit in a year when Rossi was temporarily sidelined by a broken leg.
Rossi said earlier this season that after his recent dearth of success, he could no longer be confident he was as quick as the likes of Lorenzo, Dani Pedrosa and Casey Stoner.
He told Motosprint he will begin 2013 in a similar position to Moto2 champion Marc Marquez, who will join the works Honda team alongside Pedrosa as a MotoGP rookie.
"Most of all I hope to become a problem for Lorenzo," said Rossi. "I don't know about Marquez with Dani.
"In any case we will be the number twos, and usually this type of rider has an advantage: he has nothing to lose.
"I think Lorenzo and Pedrosa will be the strongest because they are racing at such a high level. I don't yet know if I can go as quick as them.
"We'll see how the first test with Yamaha after the Valencia race goes."
He admitted he will also be casting a curious eye over to his former Ducati team when post-season testing begins, to see if the team's new recruits Andrea Dovizioso and Andrea Iannone fair better on the troubled Desmosedici than he had.
Current factory Yamaha rider Ben Spies is also Ducati-bound, but is currently recovering from injury.
"I'm very curious to try the Yamaha, just like I'm very curious to see how the guys on Ducati go, how they adapt to that bike," said Rossi. "[But] I look at myself, at how I will feel on the Yamaha."