Craig Harrison was disappointed by his side's defending after Hartlepool United's five-match unbeaten run was halted by Eastleigh.
The Spitfires, without a win in five games, took the points at Victoria Park thanks to two goals inside six minutes at the start of the second half.
Captain Callum Howe forced in the first from a corner while a well-taken Mark Yeates half-volley made it two.
Sub Jack Munns pulled one back in the final minute of normal time with a good left-foot finish from the edge of the area.
The manager said: "We had enough opportunities to win the game.
"We probably had more opportunities today than we had against Dagenham, Wrexham and Leyton Orient put together.
"The difference was in those games we defended set plays really well.
"Today our defending at set-plays was really poor.
"I've watched them back and for both Eastleigh goals we've had our set-up wrong, individuals have not done their jobs.
"That was the difference.
"If we'd only conceded the first I'd have fancied us to go on and win it."
Pools were not short of chances with Devante Rodney denied by a strong stop from Graham Stack who made two other saves from the substitute.
Munns also flashed one just wide while fellow sub Connor Simpson went over with a decent opening.
Harrison lamented those missed opportunities but, ultimately, blamed defensive failings for the first defeat since August 26.
"Devante has forced two good saves out of the keeper and Connor unfortunately has not hit target," added the manager.
"It's unfortunate they did not score, if they had that would have given us a huge shot in the arm and we could have kicked on.
"If we'd have scored then we'd have had the chance to put real pressure on.
"But we've scored in the last minute with four added on, so it was a big ask at the end."