Paul Sturrock blamed Southend's dismal first-half showing in a 2-2 draw against Crewe for ending the club's promotion dream.
Trailing 1-0 from Saturday's play-off semi-final first leg at Gresty Road, Ajay Leitch-Smith doubled the Alex's advantage in the 24th minute.
Neil Harris put the Shrimpers back in contention after the break but an 86th-minute strike from substitute Max Clayton re-established the visitors' two-goal aggregate lead, making Chris Barker's header mere consolation in a 3-2 semi-final defeat.
Sturrock said: "We can't afford to play like we did in the first 45 minutes and expect to get success.
"In the second half we had the aggression and appetite you need and if we had that from the start then it might have been a different game.
"We hit the post twice and had a goal disallowed but Crewe had chances as well and the bad goal we gave away in the first half hit us hard."