The former England Under-21s international started his professional career with Norwich City, but he failed to make the breakthrough at Carrow Road and left for Peterborough United in 2008.
A successful spell saw Lewis make 190 appearances for the Posh, playing in League Two, League One and the Championship, before moving to Cardiff City in 2012, spending four years with the Welsh outfit.
The goalkeeper played in the Premier League during his time at Cardiff, but he left the club for Aberdeen in 2016, featuring on 271 occasions for the Scottish team in all competitions.
Having only made four appearances in 2022-23, he left on a free transfer last summer, and according to the Manchester Evening News, the stopper has been training with Man United since December.
The report claims that Lewis is not on a contract at the club and largely trains with the academy side, but he has recently been used for first-team training.
Lewis allegedly took part in a session alongside fellow stoppers Altay Bayindir and Tom Heaton in the aftermath of Sunday's 3-0 win over West Ham United.
At this stage, it is unclear of the club's plans for the former Norwich youngster, but Heaton is expected to leave on a free transfer this summer, opening the door for a new number three to arrive.
Andre Onana allegedly still has the full backing of the club despite an indifferent first campaign, while Bayindir, who made his Red Devils debut against Newport County in the FA Cup on January 28, is the number two.
Lewis has not played regular football since the 2021-22 campaign, but it is understood that he has no plans to retire and will assess his future this summer.
Man United allowed Jonny Evans to train with them last summer, with the Northern Ireland international expected to remain with the academy side to build his fitness.
However, Evans's performances and presence at the training ground convinced the club to hand him a contract until the end of the 2023-24 campaign.
The 36-year-old has been a regular for Erik ten Hag's side this season, making 19 appearances in all competitions, including 14 outings in the Premier League.