The Saints were beaten 3-1 at Old Trafford despite leading for much of the second half.
United scored three in the last 15 minutes, with Amad Diallo netting an impressive hat-trick.
"Story of our season," said Ramsdale afterwards.
"We played so well for 75 minutes, but a football game's 90, 95. We had great chances, we should score more. We rode our luck massively at times and they missed chances, but if you want to get a result here you're going to have to do that.
"We've made some great blocks and then we've ended up losing 3-1. Details, mentality, just little things like that. I say little, but they're huge in football and especially in the Premier League.
"I think the goals can be stopped quite easily. I think the way we played in the first 70 minutes, that sort of scramble in midfield for the first, probably just gets turned in behind and make them face our own goal. One-twos, which we followed all night, for the second goal. And then the third's the third. Taylor's been probably the best player on the pitch. He made some fantastic passes, huge blocks and just got caught.
"So the scoreline doesn't reflect how we played, but the bottom line is we've just lost 3-1 and a game which we should have either won or got a draw out of."