The Spaniard found the net for the first time since his 37.1m move from Chelsea during his side's 4-1 win over Aston Villa.
"It feels very good, it's taken a few games but today was the day," he told MUTV.
"I felt that it was coming and, thankfully, it was today. I feel very happy and very grateful to the fans who were supporting me in the game today.
"I will try to score as many as I can between now and the end of the season and hopefully it is the first of many in my Manchester United career."
United are currently seventh in the Premier League table, three points behind fifth-place Everton.