The Red Devils are seemingly in the market for another midfielder in January having been linked with the likes of Atletico Madrid's Koke and Everton's Ross Barkley in recent weeks.
According to Portugese newspaper A Bola, Manchester United remain keen on landing Cravalho from Sporting, and sent scouts to take a look at him during his side's cup game against Porto.
However, David Moyes may face competition from a handful of Premier League rivals including Southampton and West Brom, who also had scouts at the game.
A Bola add that Bundesliga outfit Cologne had representatives at the game.
Carvalho operates primarily as a defensive midfielder, and has made 14 league appearances for Sporting, scoring twice.
Manchester United have been lacking a genuine midfield enforcer for some time - although they lured Marouane Fellaini to Old Trafford from Everton over the summer, while Darren Fletcher is on his way back after a prolonged spell out of the game with a bowel condition.