Lukaku completed his loan move back to Inter Milan on Wednesday, less than 12 months after arriving at Stamford Bridge for a club record fee of £97.5million.
And Goal reports part of the reason he left was that the Belgian felt unappreciated by Tuchel.
Whilst watching a game involving Spurs, the Blues boss offended Lukaku by joking that Conte was his "daddy". Lukaku had won the Scudetto under Conte at Inter the previous season.
It's claimed that the incident exacerbated the already-strained relationship between the pair.
The source said: "It was just a joke, but apparently, it didn't go down very well with Lukaku."