It took just 29 minutes for Dean Huijsen to give the Cherries the lead after heading home from a free-kick in front of the Stretford End.
As well as seeing Man United go 1-0 down, the goal had extra significance.
Huijsen's header was the 17th goal that Man United have conceded from set pieces in the Premier League in 2024.
That is the most goals that the Red Devils have ever conceded in a single calendar year since the Premier League began 32 years ago.
Man United pushed for a way back into the match during the second half of the contest, but their task was made even harder when Noussair Mazraoui gave away a penalty for a foul on Justin Kluivert.
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Kluivert then stepped up to make it 2-0 to the Cherries.
Things then got even worse as Antoine Semenyo made it three for Bournemouth.
Reacting to the performance, Micky Gray told talkSPORT: "Manchester United's defending has been awful today.
"I'm looking for an improvement under Ruben Amorim over the last couple of games. I'm yet to see it.
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"They've all been really poor. I can't pick out a Manchester United player who has really impressed me today. They've been really scrappy in possession, and it's all that Bournemouth deserve.
"I think Ruben Amorim has got a heck of a job on his hands here, not just in January, but the summer after that, getting players out and bringing players in."
Only Southampton (5) have conceded three or more goals in a Premier League home game more times than Manchester United (4) this season.
The game was watched by Marcus Rashford who was in attendance at Old Trafford.
That's despite being left out of the matchday squad for the third game in a row following his bombshell interview in which he said he was looking for a new challenge.
Defeat means that Man United go into Christmas Day in 13th position in the table.
It's the first time since 1989 that they've gone into December 25th in the bottom half of the table.