The 27-year-old has been heavily linked with a move to Old Trafford as Erik ten Hag looks ahead to next season.
Onana played under Ten Hag at Ajax and according to Inter's chief, steps have been taken by United to reunite the pair.
Marotta said: "Manchester United have expressed an interest (in Onana) which may or may not lead to an offer in the next few days.
"Once that happens, both Inter and the player will carefully assess it."
Of course Onana's future hinges on the contract drama surrounding David de Gea.
MORE ON MAN UNITED
Man United must sell to finance Andre Onana move as David de Gea talks rumble on
De Gea sends cryptic message as Man United future remains unclear
With June 30 being the key date, the Spaniard is now desperately seeking a solution to his future.
Manchester United released a statement on Friday evening stating that talks were ongoing over De Gea's future, but according to Man United fan and presenter Flex, the whole saga has left a bitter taste in his mouth.
He told The Transfer Insiders: "It's not nice man, in all seriousness.
"Some Manchester United fans don't care about this side of it, or some that I've spoken to anyway, but I do.
Most read in Football
West Ham 'would not be a step up' for Zaha, but Tottenham switch would be 'exciting'
Liverpool agree Szoboszlai deal, De Gea latest, Mount fee agreed, Ziyech move off
Cundy gutted by 'catastrophic' decision to let Mount leave Chelsea for Man United
Liverpool agree deal with RB Leipzig for Szoboszlai after triggering release clause
"I'd have liked to have seen this handled better. I think that you can be ruthless, but you can be respectful and you can be organised.
"Both sides probably haven't covered themselves in glory with the way that this situation has gone and then you add in the fact that as the season went on Erik ten Hag just changed his opinion."