The Belgium international was in the best shape of his career during his time in Italy, adopting a strict Mediterranean diet.
The 29-year-old scored 15 goals in all competitions, but only eight of those came in the Premier League.
Lukaku has since re-joined Inter on a season-long loan and according to Gazzetta dello Sport, they want him back in peak physical condition.
A target weight of 101kg (15st 9lbs) has been set and that is what Lukaku weight at the end of the 20/21 season.
That year saw the former Everton man score 24 times in 36 appearances to help Inter to the Serie A title.
Lukaku’s diet consisted of salad, fish, chicken and shirataki pasta. Off the menu are potatoes, fried food and mozzarella in addition to alcohol.
It allows him to be more explosive, compared to his diet in England which focused on him becoming bulkier.
He told Sky Sports Italia during his first spell as an Inter player: “Since I joined Inter I have changed my diet and I can say that I have never felt so strong.
“My diet consists of salads for lunch, a lot of chicken breast, Shirataki pasta.
“I don’t change my way of eating much, for how we play we must have a very strong physique, we run a lot: since I have followed this lifestyle I feel better on the pitch, I feel more reactive and faster.’
“I’ve always been an explosive player with muscles but Italy took me to another level. I have never felt so strong. I have reached another level, physically and mentally.
“I consume a lot of carbohydrates after games. Most of the time I eat two packs of gnocchi, it depends on what the chef has prepared. They go into circulation quickly and they help me a lot.”