Niall Currie was left "flabbergasted" after Portadown's relegation to the Championship was confirmed on Saturday.
A 3-2 defeat to Dungannon Swifts ended the Ports' hopes of staying in the Premiership this season.
It was a dramatic afternoon at Stangmore Park, with Currie's side trailing 1-0 and 2-1 before Michael O'Connor's 79th-minute goal earned Dungannon a 3-2 win.
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Currie blasted some poor defending which contributed to Dungannon's goals, and believes his side dominated the second half at Stangmore.
He told BBC Sportsound: "I am still flabbergasted after dominating the second half like that and coming away with nothing. We came in at half-time 1-0 down after not starting the game for the first 30 minutes.
"We got ourselves into the game in the last 10 minutes of the first half by just forcing Lee Chapman to play like a winger instead of like a right-back.
"And he got at Dean Curry. He got past him, got past him and got past him so we told him to do the same thing in the second half.
"I thought Lee Chapman was outstanding today."
Reflecting on the three goals his side conceded, Currie said: "We dominated the whole second half but ultimately, look at the goals we concede yet again.
"Free header five yards out from a corner kick. A designated player.
"The second is something you wouldn't see in a BB League, if we're being perfectly honest. Ball lumped up 50 yards with two red shirts alongside JJ (Jordan Jenkins) and it travels into the keeper's area and he goes back onto his line and JJ knocks the ball in from 15 yards.
"The third is, again, us not picking a player up and (Michael) O'Connor gets in between two red shirts to tap home from close range.
"Again, we had good saves from Declan Dunne. If I am being absolutely honest, and this is the biggest sickener, I thought we battered them in the second half.
"The old Achilles heel. Since we came to the club we have looked like we can score a lot of goals. I think when you come away from home and score two goals you get something out of the game.
"But you don't get anything when you defend the way we did."
Currie will now have to plot Portadown's return to the Premiership, but accepts he could have a battle on his hands to retain most of his squad for a year in the Championship.
"If we keep that squad together for the Championship we won't be too far away. But most of our players are out of contract for a start, and there will be players who want to stay in the Premiership," he said.
"It will be their decision what they want to do. We know what we want to keep, and if we keep that squad we won't be too far away next season.
"But regardless of what happens, we won't be far away anyway."
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