United boss Amorim began the season with question marks over who would be his first-choice goalkeeper.
Bayindir’s error against Arsenal on the opening day only added to that, and despite playing all three Premier League games this season, another mistake against Burnley on Saturday put the club under pressure to sign a goalkeeper.
Last season’s number one, Andre Onana, has had a turbulent two years at United since his £47.2m transfer from Inter Milan in 2023.
His only appearance so far this campaign came in the cup exit last week, when he saved only one of 12 penalties in the decisive shootout.
Former Manchester United midfielder Nemanja Matic called him “one of the worst goalkeepers” in United’s history before a 2-2 draw against Lyon last season – with Onana again at fault for both goals.
Amorim elected to drop the Cameroon international for the next game against Newcastle United.
Lammens’ imminent arrival adds to a busy deadline day at Old Trafford.
Antony is expected to complete a move to Real Betis, Jadon Sancho is set to head to Aston Villa on loan and Rasmus Hojlund is on his way to Napoli.
However, Kobbie Mainoo is expected to stay at United.
The England midfielder, 20, wants a loan move after seeing his appearances limited.
Mainoo, who has been left out of Thomas Tuchel’s latest England squad, featured as a substitute in United’s 3-2 win over Burnley on Saturday.