The Nerazzurri have been linked with Lukaku in recent weeks, but Marotta suggested the financial implications of the deal must be properly accounted for. When asked about the United frontman, the Inter chief told reporters:
"The transfer market is a work in progress. We are working hard to get the best squad possible for the new season. We are ambitious and want the best team possible. We also must find ways to respect all financial perimeters too.
"We cannot rush things because that can lead to mistakes. We have clear ideas, but we need to be patient. We have important goals and transfer targets in mind."
Lukaku moved to Old Trafford in 2017 for an initial £75 million, but the Belgium international hasn't been the success many at United would have hoped for and could be sold this summer.
The links with Inter have been strengthened after Antonio Conte was appointed their new manager this summer. Journalist Kristof Terreur recently noted Conte had a serious interest in Lukaku (then at Everton) when he was manager of Chelsea in 2017:
Lukaku also encouraged speculation in June when he told Sport Mediaset (h/t Evening Standard's Vaishali Bhardwaj) Conte was "the best manager in the world" and that he wanted to play in Italy:
"I was always a big fan of the Italian league. People close to me, they know I have two leagues that I always dreamed about—the English league and Serie A. So, for me, Serie A is my favourite league as well.
"The transfer of [Cristiano] Ronaldo, the Italian league is now coming back. Now Antonio Conte going to Inter Milan, which is really good because he is, to me, the best manager in the world.
The 26-year-old scored 15 goals in 45 appearances across all competitions last term and has enjoyed patches of success. James Robson of the Evening Standard wrote the Red Devils are also open to keeping their man:
Marotta's comments suggest the two parties could still unite this summer, though Lukaku has joined the rest of United's squad on their pre-season tour of Australia.
Mauro Icardi's potential departure could free up the funds and squad space necessary to accommodate Lukaku's arrival. Marotta also confirmed Icardi and midfielder Radja Nainggolan "are not part of our project," with each likely to be sold this summer.
Lukaku enjoyed a fruitful first season at United and returned 27 goals in 51 appearances. Sky Sports Statto recently pointed to his strike rate as one of the club's best in recent times, having joined around 18 months after team-mate Marcus Rashford made his debut:
Inter seemingly have a strong desire to comply with financial fair play regulations, though a drawn-out transfer saga could also work in their favour if they want United to drop their valuation of Lukaku.
The Premier League transfer window will close on August 8, while Serie A clubs have a couple more weeks to make signings and won't see their window shut until August 23.