Morgan was leading the side in the absence of Alastair Cook, who was ruled out on the morning of the game with a groin strain, and presided over England's heaviest ODI defeat on home soil and their sixth-lowest all-out total anywhere.
England had won four ODIs on the bounce before this match but now they go to Old Trafford with their tail between their legs.
"It's as badly as we can bat," Morgan said. "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.
"It was a little bit tacky, and nipped around a small bit - but we're good enough to negotiate that. 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."
Morgan then tried to spin the margin of defeat as meaning England will be able to move on more quickly from the result than had it been a narrow loss.
"I don't think it's going to be a huge setback," he said. "I think it'd be a bigger one if we fell 40 runs short, had substance throughout the whole innings, looked like we were going to win.
"This is a performance I think we've got to draw a line under, and look forward to Manchester. You can't look into it too deeply - you'd be here for weeks. But today, throughout our batting display, it was just terrible.
"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."
It remains unclear whether Cook will be fit for Old Trafford on Wednesday to try and pick up the pieces from this defeat and ensure another damaging run of results does not emerge.
"I'm not going to dwell on it a hell of a lot," Morgan said, "but certainly as a batting unit, there are a few guys who need to up their game a little bit. With the series in the balance, we've got to do that for Manchester."
Angelo Mathews, meanwhile, will be a far happier captain travelling south west on Monday. The team were forced to train indoors on Saturday and plenty were still sporting multiple layers against the spring chill.
"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. Yesterday was a great example, we trained really hard and conditions weren't good, we had to train indoors, but the focus was there and it paid off."