Our American number one pulled six saves out of the top drawer to keep the title favourites at bay until he was finally beaten by Paul Scholes in the 68th minute.
Indeed, the first half was the Keller v Van Nistelrooy show with Kasey denying the flying Dutchman four times. However, Van Nistelrooy had the last laugh with United's second goal in injury time as the Premiership leaders ran out 2-0 winners.
That was harsh on Kasey, who admitted: "It was a good game but I don't try to categorise my best performances when we've lost.
"I had a good game, yes, but not good enough. I felt good, felt I was moving well and getting myself in the right positions so that felt good - it was just a shame the way it finished.
"You make a few saves and you think okay, maybe we'll come back, get that goal and ride our luck but we came up against a very good team in very good form and we weren't able to turn it around.
"Obviously you take pride in your own performance and the good and bad about team sports is when you are not having a good day, people can cover and sometimes when you have a good day, you can't do enough.
"That is the frustration. But even after a good performance, you aren't happy if you have lost the game. You shouldn't be happy either."
Kasey Keller v Manchester United
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Kasey Keller conceded his overriding feeling was one of bitter disappointment despite his super show against United on Sunday.