Morgan was satisfied with a target of 256 for eight at Chester-le-Street, where England were hoping to go 2-0 up in the Royal London one-day international series.
Instead, they collapsed to 99 all out - their lowest ODI total in this country since 2001 - en route to their widest margin of defeat on home soil in this format.
Morgan's 40 was one of only two double-figure scores as England were bowled out in 26.1 overs, in a series now all square with three to play.
"Today, throughout our batting display, it was just terrible," said Morgan, who agreed England are probably incapable of playing any worse.
"I think so. It's as badly as we can bat anyway.
"It was one of the worst performances I've been a part of, in an England shirt.
"I can't explain why we played that badly."
Sri Lanka beat England at their own game, Tillakaratne Dilshan (88) and then Ashan Priyanjan eking out a near par score in conditions favouring the bowlers, and Nuwan Kulasekara (three for 15) then producing a spell of three wickets for one run before Sachithra Senanayake (four for 13) cashed in with new career-best figures.
England were initially optimistic they could chase the target.
"It was a little bit tacky, and nipped around a small bit - but we're good enough to negotiate that," added Morgan, who had earlier on Sky Sports described his side's performance as "abysmal".
"It's a pitch we would have turned up at and said 'Yes, perfect, we'd love to play' - especially against a side like Sri Lanka. But we've been outplayed.
"We bowled quite well at them. We probably lacked a few wickets, but we still contained them to four or four-and-a-half an over - which was a great effort - and we were happy with the score they got.
"But we lacked partnerships through the whole innings, and never seemed to get going. We just struggled."
England did not approach the task badly, according to Morgan, but on this occasion their skills against the moving ball simply were not up to it.
"I don't think it lacked any purpose.
"It just lacked enough skill to negotiate good balls."
They were found wanting too in the field, dropping two routine outfield catches.
"We practised hard yesterday, and I can't recall any catches we dropped (then) at all," said Morgan.
"So that's becoming a huge annoyance.
"The quicker we start taking our chances the better. But even still, we were happy with that score."
Morgan's opposite number Angelo Mathews had plenty to smile about, after his team had recovered so completely from their opening defeat at The Oval three days ago.
"It always feels better to beat England at home, because they have a very strong side when playing at home," he said.
"We had to dig deep after the loss in the first one-dayer.
"Nuwan Kulasekara was unplayable.
"In those conditions, he's a nightmare for the batsmen. He swings the ball both ways, and has done it so many times for us."
Morgan, meanwhile, is seeking consolation in England's win in London - and hoping for a return to those standards in Manchester on Wednesday.
"If you look at the quality or skill of the players, in the last game, you'd be really happy with it.
"Today, it was the whole game ... so it's easier to wipe to one side. I don't think there's a huge amount of change that needs to be made."
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