The Red Devils had scouts present as Valencia faced Athletic Bilbao at Mestalla at the weekend, and they reportedly watched players from both camps.
Super Deporte has suggested that los Che full-back Juan Bernat was the main reason for their visit, with David Moyes keen to lure the youngster to Old Trafford in the summer.
The Scottish manager is believed to rate the 21-year-old highly and wants to sign him as a long-term replacement for Patrice Evra.
Meanwhile, Manchester United scouts also made a check on long-term target Ander Herrera, who has played a major part in helping los Leones close in on the final Champions League berth in La Liga.
The Red Devils have finally raised their form of late, but they are still 29/20 underdogs to see off rivals Liverpool at Old Trafford next weekend.