Manager Sir Alex Ferguson is actively seeking another keeper amid fears that Polish star Tomasz Kuszczak, 29, will quit United after losing out to Ben Foster.
Despite Ferguson's claims Foster will be his long-term No 1, doubts are beginning to creep in at Old Trafford.
Earlier this week United's goalkeeper coach Eric Steele admitted that the club were on the lookout for more competition.
United are also watching Schalke 04's highly-rated keeper Manuel Neuer as they prepare for what could be Edwin van der Sar's final season.
Neuer, who is 23 and 6ft 4in, has one senior cap and starred in Germany's European Under-21 championship-winning side.
Patricio, 21, has emerged as an alternative and United will continue to monitor his form this season.
The 6ft 2in, Portuguese stopper comes straight out of the same school as Great Dane Peter Schmeichel - scoring with a header against FC Twente to put Sporting through to this season's Champions League knockout stages.
Although Fergie hung his hat on England keeper Foster, 26, being first choice, his highprofile errors against Chelsea in the Community Shield, Arsenal and Manchester City are causing concern.
Or, Manchester United will offer Inter Milan £20m for Brazilian keeper Juilo Cesar. (Sunday People)
Rui Patricio's agent claims Manchester United have been watching the Sporting Lisbon goalkeeper for the past couple of seasons.
The 21-year-old is highly rated and has reportedly been a target for United from the time Carlos Queiroz was Sir Alex Ferguson's assistant.
Queiroz left Old Trafford in 2008 to become Portugal's national team coach and Patricio is a regular member of his squad.
Reports are claiming United are lining up Patricio as a potential long-term replacement for Edwin van der Sar, who is tipped to retire in the summer.
Patricio's representative Carlos Goncalves confirmed the Premier League champions have been scouting the Sporting shot-stopper.
Goncalves told A Bola: "It is true that he has been watched, ever since Queiroz was with Manchester United."
Though Patricio has yet to make his senior debut for the national team, he was also part of former coach Luiz Felipe Scolari's squad for Euro 2008.
And the promising shot-stopper made his Champions League debut against United almost two years ago in a 2-1 group-stage defeat at Old Trafford.
Manchester United close in on Sporting Lisbon stopper Rui Patricio on the advice of Cristiano Ronaldo
Safe pair of hands: Sporting Lisbon's Rui Patricio
Manchester United are keen on the Sporting Lisbon goalkeeper Rui Patricio, according to his agent.
Cristiano Ronaldo is understood to have recommended Patricio, 21, to Sir Alex Ferguson after United’s 2-1 Champions League win over Sporting in September 2007, and the United boss has received a string of glowing reports from his Portugal-based scout Toninho Cruz.