Lukaku returned to Stamford Bridge from Inter in a big-money summer transfer after two successful years in Italy, committing to a five-year contract with the Blues, goal.com reported.
However, the 28-year-old has not been an automatic starter under the German as he has dealt with a string of fitness issues, and has risked the wrath of club officials by expressing his frustration in the Italian media.
Lukaku sat down with Sky Italia to discuss his situation at the start of December, and admitted to being unhappy in west London.
The Belgium international also hinted at a possible return to Inter, saying: “Physically I'm fine, even better than before. After two years in Italy, in which I worked a lot at Inter with coaches and nutritionists.
“I’m not happy with the situation [at Chelsea] and that’s only natural. The head coach has decided to play a different system but I need to keep working hard and be professional.
"I’m not happy with the situation but I’m a grafter and must not let up."
Lukaku's remarks have come as a shock to Tuchel, who claims to have seen no signs of any dejection or anger from the forward in training.
Speaking ahead of Chelsea's top-of-the-table clash with Liverpool on Sunday, the German head coach told reporters: "We will talk with Lukaku openly behind closed doors. I am surprised because I don't see him unhappy – the opposite.
"We here can take the time to try to understand what is going on with Romelu, it does not reflect the daily attitude."
Tuchel says the publication of the interview is an unwanted distraction for his side, but he is reluctant to criticise Lukaku at this stage in case his words have been misinterpreted.
"We don't like it of course. It brings noise that we don't need and it's not helpful," he added.
"We don't want to make more out of it than it actually is. It is easy to take lines out of context, shorten lines, make headlines and then realise later that it is not so bad and maybe not what he meant."