After beating South Africa at The Oval in London last Thursday to get their campaign off to a winning start, the hosts and pre-tournament favourites narrowly failed to reach the huge 349-run target set by Pakistan in Nottingham.
Their fielding in particular was scrappy, with preventable boundaries and an important but relatively easy catch dropped by Jason Roy.
"We've gone from probably one of our best performances in the field at The Oval to not extremely bad, but it's cost us probably about 15 or 20 runs in the field, which is a lot in a one-day game," Morgan told reporters.
"Fielding is an attitude thing, so it's a matter of taking maybe our stand-off attitude today that we had in the field and getting it back to our positive attitude: going for everything, playing with that fearless nature, maybe taking a half chance instead of standing off a normal chance.
"You can play against a side that's better than you or just have an off day. Same with the ball. But when it comes down to the difference in the sides being in the field, it's even more frustrating."
Nevertheless, Morgan felt the side had done a good job with their bowling and said the 130-run partnership between Joe Root and Jos Buttler had kept the game alive.
"They never really got away with us given that the wicket was good and the outfield was very fast. And when we recovered with the bat with that substantial partnership, we were never really out of the game probably until the early 40s over," he said.
"There are some really good positives to take out of our batting and our bowling."
England next face Bangladesh on Saturday in Cardiff.
(Reporting by Hugh Lawson; Editing by Ian Ransom)
