Since his return to Ferrari, the Finn has failed to mount a challenge in the opening four races of the Formula One season.
The 34-year-old has not managed to finish higher than seventh, and has amassed just 11 world championship points for the season, 68 behind leader Nico Rosberg.
While Raikkonen has struggled, his team-mate Fernando Alonso appears to be extracting more from the car, securing a third-place finish last time out in China.
Lopez admits Raikkonen's display for his new team have been surprising, but backed him to recover once he settles back into life at the Italian manufacturer.
"Yes, definitely (I am surprised)," Lopez told Auto Motor und Sport. "With us he was a strong rider, so the gap is a bit surprising.
"But I also know how good Fernando is.
"But we also know how good Kimi is. It is a new team for him and with Kimi a lot depends on what is happening around him.
"With us he was always fully comfortable, even though of course I cannot exactly know why things are not going right for him."