Lukaku left Inter in August and returned to Chelsea for a second stint in a deal worth a club record £97.5 million.
"After two years in Italy, in which I worked a lot at Inter with trainers and nutritionists, I am physically fine. But I'm not happy with the situation, this is normal," Lukaku told Sky Italy.
"I think the coach has chosen to play with another system, I just have to not give up and continue to work and be a professional. I am not happy with the situation, but I am a worker and I must not give up."
Sky did not say when the interview took place and did not elaborate on the 'situation', with the full interview set to air on Friday.
Lukaku has either led the line or played alongside a fellow striker this season at Chelsea but the Belgium international has missed nine games since October due to injury and COVID-19, scoring only seven goals in all competitions so far.
Chelsea were top of the standings for several weeks but now find themselves in second place, eight points behind leaders Manchester City after two wins in their last six games.
Lukaku revived his career at Inter where he scored 64 goals in two seasons and won the Serie A title before his departure. The 28-year-old said he hoped to be reunited with the club in the future.
"I really hope in the depths of my heart to return to Inter, not at the end of my career, but at a still good level to hopefully win more," he said.
"I have always said that I have Inter in my heart, I will return to play there, I really hope so. I am in love with Italy."
Chelsea host Liverpool on Sunday with the Anfield club one point behind Tuchel's side with a game in hand.