The seven-time champion won twice and claimed 10 podiums during his final year with Yamaha in 2010, finishing third in the championship.
Following his switch to Ducati he has experienced the first two winless seasons of his MotoGP career, with just three podiums - only one of them in the dry – to boast.
Rossi said in November that he had learned nothing during his barren two-year spell with the Italian marque.
Lorenzo believes Ducati's 'complicated' bikes have been key to Rossi's recent struggles, predicting that the Italian could once again become a frontrunner with Yamaha.
"Its true, the Ducati bike seems to be very complicated," Lorenzo admitted earlier this month at the Race of Champions.
"Only Casey [Stoner] won a championship and races with it, so for sure it's not easy.
"Vale is a strong rider, and really complete, but he couldn't [win] with it.
"Maybe with Yamaha he returns again to a high level."
Lorenzo reiterated his initial reaction that Rossi's return to Yamaha was a boost for both himself and the team.
"Vale coming again to Yamaha is good for the brand because we [will be an] even more competitive team than last year," he said.
"He has a very high motivation to keep fighting for wins, so I'm really looking forward to his performance on the Yamaha."