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.
Emiliano Martinez might be an option for Man United's problems, but it won't last long because Martinez is no longer young, and from a business perspective, it is also less promising because Martinez's price is still quite high, and it is clear that his transfer value will decrease every year after joining Man United. This may have been a tough decision for Amorim as he opted to go ahead with the speculation by signing Senne Lammens.
Senne Lammens is indeed young and much cheaper than Martinez, but he has no previous Premier League experience, and there's no guarantee he'll perform better than Onana or Bayindir.