Birmingham head coach Lee Bowyer had mixed emotions after his much-changed team squeezed into the second round of the Carabao Cup with a 1-0 win over League Two Colchester.

Marcel Oakley's 76th-minute debut goal clinched victory for the Sky Bet Championship side after Colchester created the best chances and later hit the bar through Cameron Coxe.

Bowyer changed all 11 players from Saturday's 1-0 win at Sheffield United and the Blues boss was relieved his second string passed a stiff test.

"What we practise every day and bring into the game we didn't do in the first half and that disappointed me," said Bowyer.

"I want players to show me I was wrong in not picking them on Saturday and they didn't do that."

Bowyer believes right back Oakley, who made his debut at 18, took his chance, however.

"We had to use Marcel in pre-season because we haven't got cover for Maxime Colin," he added.

"Marcel is only 18 but he has a big heart. I don't give debuts away cheaply but he deserved it based on his pre-season. I knew I could trust him."

Colchester head coach Hayden Mullins – an ex-West Ham United team-mate of Bowyer's – felt hard done to by the result after on-loan Fulham forward Sylvester Jasper threatened three times.

"I'm disappointed with the result, which is a surprise for a League Two club coming to a Championship outfit" said Mullins.

"We dominated most of the game which is really good. I wanted us to be positive and aggressive with the press and in our attacking play and not show them too much respect and we kept pushing right to the end and hit the bar.

"There were a lot of positives – the way we pressed and the energy we showed, I don't think many teams in our division would have lived with us."