With Ferrari's title hopes high this year, team chiefs issued a warning about Raikkonen's performance after Shanghai, amid rumours that Daniel Ricciardo or Sergio Perez might be in the frame for his seat for 2018.
"I haven't heard anything about what his problems have been, but Kimi did not have a very easy start to the season," Lehto, a former Ferrari test driver, told Finland's Iltalehti newspaper.
"But the team is working really well and I know Kimi can do better.
"When you start having problems, it can snowball quickly but once you get the small details sorted out things can start to work well again.
"After all, Kimi has definitely not lost his speed. Last year he drove really well and that doesn't suddenly disappear over one winter."
Raikkonen is currently fourth in the driver standings with 34 points, half the tally of teammate and championship leader Sebastian Vettel.