After cancelling out Steve Mounié's first-half header, former Town loanee Lukas Jutkiewicz then had an effort ruled out for offside in the second period.
And after the 1-1 draw, Cotterill felt it was a marginal decision which could have seen his side walk away with more than a St Andrew's replay in 10 days time.
"It's a real tight one and we're slightly disappointed with that because getting a win would have been a fantastic result for us,” Steve Cotterill told the press after the game.
"It's a great result anyway. We played well, especially making seven changes and great credit to those players coming in, their fitness levels were tremendous.
"It gives everyone a lift to not get beat. That's the big thing. The club, over the last year or so now, has probably got used to getting beaten every week."
With both sides having important league games on Tuesday, with Liverpool visiting the John Smith's Stadium and Birmingham playing SkyBet Championship relegation rivals Sunderland, wholesale changes were made to both teams' starting XIs.
But it was Cotterill who was the much happier of the two coaches after the encounter and was delighted with the result and the performance of his second-string side.
"I'm not really surprised that the lads have gone out there and put a shift in like that because I see them do that in training," Cotterill added.
"I'm really pleased for them. We've done a lot of work, a lot of preparation coming into this game - as much as we've done with the team that have been playing in the league.
"There were no corners cut, no excuses going into this game and they executed the plan brilliantly."