United hit a remarkable - for all the wrong reasons - new low in west London as goals from Josh Dasilva, Mathias Jensen, Ben Mee and Bryan Mbeumo saw the Bees four goals to the good at half-time.
It got no better after the break for United and manager Erik ten Hag, who has now lost both his opening two fixtures in charge after last week's home defeat to Brighton.
Ronaldo returned to the starting line-up after being left out at Old Trafford a week ago, amid a public desire to leave the club just a year after completing a sensational return.
According to Manchester Evening News' Samuel Luckhurst, United assistant coach Steve McClaren urged Ronaldo at full-time to head over to the travellings fans but, after an exchange of words, opted to instead head to the dressing room.
McClaren advising Ronaldo to go over to the fans but Ronaldo not having it. Exchanges some words with him and then heads for the tunnel. #mufc
— Samuel Luckhurst (@samuelluckhurst) August 13, 2022
Some players did go over, but were largely booed. Captain Harry Maguire did acknowledge the fans though without making too much effort to approach before following Ronaldo down the tunnel.