Lukaku has replaced Icardi at the head of the Inter attack, signing a five-year contract with the Serie A club in a reported €80million transfer from Manchester United in August.
He has scored only three goals in eight appearances but Icardi, who last month left Inter for Paris Saint-Germain in a loan deal that can be made permanent, does not believe Lukaku has endured a poor start to life in Italian football.
"It doesn't seem to me that he's experiencing a moment of difficulty," Icardi told Gazzetta dello Sport. "In fact, I think he's started well, scoring a couple of goals immediately.
"It's not easy for anyone to play in Italy. Serie A is the most defensive championship in Europe.
"Maybe he needs a little more time to settle in, but he's already scored a lot of goals in his career. And he will do it again."
Icardi is not focused on what his future may hold, instead concentrating solely on his form for PSG. The Argentina international has scored two goals in four games for the defending Ligue 1 champions.
"This year, I'm at PSG and my goal is to give my best for this shirt," Icardi said,
"Then at the end of the season, around May or June, we'll see what happens. It's still too early to say anything."