Despite having arguably the best driver – Fernando Alonso – at the wheel for the past two seasons, McLaren has struggled notably with Honda power.
But Rosberg, who in 2016 won the title for Mercedes before retiring, said: “Fernando will be on the podium this year or very close to it.”
Speaking to the Spanish sports daily Diario Sport, the German said that is possible because the 2017 technical regulations mean F1 is “starting from scratch”.
“He (Alonso) has a great chance but we will have to wait, because the same thing can happen with other drivers and teams too,” Rosberg added.
“But I do think McLaren will significantly improve its performance. I don’t expect them to dominate, but they have a new opportunity and they will definitely take advantage of it,” he said.
However, Rosberg said he thinks his old team, who have replaced him for 2017 with former Williams driver Valtteri Bottas, will still be strong this year.
“I don’t think it’s going to change,” he said, referring to Mercedes’ recent dominance. “It’s a great team that works like an oiled machine.”
As for how Ferrari will do, Rosberg plays down the Maranello team’s chances of moving in front of Mercedes or even Red Bull.
“Maybe they will improve but I don’t see them leading the championship,” he said.
“I think the main danger (for Mercedes) will come from Red Bull,” Rosberg explained, “because the marriage between Adrian Newey and new regulations has always worked extraordinarily well over the last 20 years or so.
“We’ll have to wait, but we’ll know soon,” he said.