The pair rode alongside each other from 2008 to 2010, with Lorenzo besting the Italian and clinching the MotoGP championship in what was Rossi's final year before his move to Ducati.
The Italian has not won since however, and currently lies eighth in the championship.
Lorenzo, in comparison, won three times last year and already has five victories to his name - as well as a 23-point advantage in the championship.
"It's a big motivation to have him back because I always like to have the best riders in my garage," Lorenzo told the BBC. "Valentino is one of them. I assume trying to beat me will be a big motivation for him.
"It's [also] good news for Yamaha because we achieved a lot of good results together. That's the most important thing."
Lorenzo added that the Italian should not be written off as a result of his two fruitless seasons with Ducati, saying: "Someone who has talent normally keeps it and doesn't lose it like this in two years.
"He is still young enough. He is 33, he has won a lot of races. With the Yamaha a better bike this year, it will be easier for him.
"It's true that us younger riders are getting stronger, more experienced and more consistent, but we have to remember that Valentino has won a lot of races.
"I think our relationship has got better, though maybe it's got better because we were in different teams. We are both winning riders and neither wants to lose. We have to respect that and keep it in mind."
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