The no.31 has been worn by some big names in the past, including Nemanja Matic, who had it throughout his United career, and Bastian Schweinsteiger.
Marouane Fellaini held it for two seasons, before switching to no.27, while it was the first number that current Under-18s coach Darren Fletcher wore when he was promoted to the senior squad by Sir Alex Ferguson.
Lammens will actually be the fourth different goalkeeper to be assigned the no.31.