Lammens has signed a five-year contract with United, who have been struggling to find consistency in goal. He has been brought in to replace Andre Onana and Altay Bayindir, who have had a difficult start to the season.
Lammens was impressed by the club's stadium and the legacy of its goalkeepers. He spoke about his decision to join United, saying it was a childhood dream come true.
He said: "When I was five, United were at the height of football. You keep in your memory how big Manchester United is. When I heard they were interested, it was like a childhood dream. The club alone, the aura of the club, it's massive."
Lammens is looking forward to making his debut for the club and contributing to its history. He is determined to make a positive impact and help United improve its performance in goal.