Lukaku has signed on a season-long loan from Chelsea after a year away from Inter.
Pellegrini told TMW: "All of us Interisti are happy for his return.
“For me he's the best centre-forward in the world. I can't think of a better one.
“Out of players from my Inter, he reminds me of Aldo Serena, a striker who can get in behind and has a great sense for the goal.
“For the Scudetto we must also see how the other teams strengthen," Pellegrini said, “from Juve to Milan. It's premature to say, there are also Napoli and Roma, and we need to see how they bolster their teams. It's too early for predictions and rankings."