David Moyes is believed to have sent scouts to watch the 21-year-old in action a staggering twelve times already this season.
The Scot has been left frustrated in his pursuit of various midfield targets, and has been meticulous in running the rule over the Portuguese international.
Scouts were present to watch Carvalho play his part in Sporting’s 1-0 win over Olhanense at the weekend, and were once again left impressed by his midfield performance.
United want to take their time to decided whether or not to make a summer offer for Carvalho, especially since he is likely to cost the club over £30million.
He is considered one of the rising stars of European football, and also plays in a position in which Moyes remains desperate to strengthen.
Central midfield has remained a problem position for Manchester United all season, with the likes of summer signing Marouane Fellaini struggling for form and fitness.
Moyes’ side currently occupy seventh place in the Premier League and are facing the genuine possibility of missing out on the Champions League for the first time in almost two decades.