Cristiano Ronaldo thanked Manchester United fans for a "massive and unbelievable reception" but was sad to see his former side eliminated from the Champions League.
Ronaldo scored Real Madrid's winner in their 2-1 victory to prevail 3-2 on aggregate in the round-of-16 tie. He however opted for a muted celebration in recognition of his six years with United.
The apologetic response to poking in Gonzalo Higuaín's cross was reminiscent of his goal for United at Sporting Lisbon in 2007, when the home fans applauded one of their youth team's greatest products.
"Thank you for a massive and unbelievable reception. I'm happy to win but also sad for Manchester United," the Portuguese tweeted on Wednesday morning.
Prior to the match, the reading of the line-ups unusually began with United when usually the away side's matchday squad is announced first. Ronaldo's name was bellowed last to a cacophony of cheers, prompting the 28-year-old to make a heartfelt gesture to United fans.
He admitted after the game he felt "shy" playing at Old Trafford for the first time in nearly four years.
"I'm never going to forget this moment. It's unbelievable. I feel happy for Madrid to go through but I feel a little bit sad," Ronaldo added.
"It's quite strange and I felt a little bit shy. It's a feeling you cannot explain."
Ronaldo entered the United dressing room after the game - as he had done at the Bernabéu three weeks ago - before he left the stadium.