Manchester United have decided to sign Senne Lammens and it is now clear that they are no longer counting on Martinez. I think Martinez would have been a much better goalkeeper for them, but Manchester United have made a lot of strange decisions in recent years, so this decision is not surprising.
I think the reason is the age factor. Emiliano Martinez is 33 years old, so spending a lot of money to sign him doesn't seem like an ideal thing for the club to do. He definitely has more experience, he has been playing in the EPL for a long time, but i believe United were looking for something long term.
I do not know much about Senne Lammens, but he is young and he arrives with quite a reputation, he could be a success at United. That said, i think this transfer relegates Onana to third choice goalkeeper, from first choice last season to third choice this season, he might have to leave in January.