Last week, 90min reported that United had begun negotiations with Inter for Onana as the likelihood of David de Gea leaving Old Trafford increased.
Now that Marotta has acknowledged United's interest in the 27-year-old, he has emphasised the urgency of making a formal offer.
Manchester United has indicated interest in Onana, but that desire must result in an offer or it is not real, according to Marotta. "After that, careful evaluations by the player and us will take over."
United must decide shortly whether to launch a bid for Onana as well as how to handle De Gea's employment situation.
De Gea agreed to a contract extension that would have reduced his compensation by about half, to £200,000 per week, but United later withdrew that offer and tried to persuade the custodian to accept an even bigger paycut.
Since that most recent offer has yet to be accepted, De Gea will soon be a free agent after his contract with United expires on Friday night.
Even if De Gea can be persuaded to stay at Old Trafford, United may still want to sign Onana as manager Erik ten Hag looks for goalkeeping competition of the highest calibre.
