United signed Uruguayan full-back Guillermo Varela this week, and could follow that up with the 17 million-pound capture of Argentina international Garay, the Mirror reports.
According to the report, an agreement in principle is in place between the new Premier League champions and the Lisbon giants for the 26-year-old.
The report further said that although Rio Ferdinand has signed a one-year contract extension, he would turn 35 later this year.
There are ongoing doubts over the future of United skipper Nemanja Vidic, and with Chris Smalling and Phil Jones plagued by injuries last season, new United boss David Moyes wants defensive reinforcements, the report added. (ANI )