Alan Stubbs said his Everton Under-21s side must learn to focus right until the final whistle after watching them lose late on against West Ham United on Monday night.
The Blues' second-string were playing their first home game of the 2013/14 campaign but left Southport's Merseyrail Community Stadium pointless following a 2-1 defeat.
Luke Garbutt's stunning strike had cancelled out Callum Driver's freak opener but a mix-up one minute from time allowed Sebastian Lletget to nip in and snatch all three points.
"I thought it was a good game and a really competitive game," Stubbs told evertontv. "They put lots of balls in the box, they got players forward, they played on second balls, which is what you associate with Big Sam [Allardyce]'s teams.
"I thought we got good blocks in and we got back into it with a great goal. But we made a mistake which has given them the winner and that can't happen.
"The players have to learn that when you're getting towards the end of the game you don't take chances and, worst case scenario, we would have drawn the game 1-1. But their winning goal has come from a mistake and that's always disappointing.
"It's early stages and we'll evaluate what we did well - which will be a lot of points - and what we didn't do so well, which there won't be too many."
Next up for the Under-21s is a trip to Manchester City on Monday 26 August.