Warnock, suffering from a virus which has affected the club, listened to the match on the radio and kept in touch by phone with assistant Mick Jones and coach Ronnie Jepson.
With Birmingham leading through Wade Elliott’s 32nd-minute strike, Warnock ordered El-Hadji Diouf and Sam Byram to be brought on, and Luciano Becchio equalised after an hour to earn a replay.
Jones said: “Neil was in constant touch with us during the game. He made the decision to change the team at half-time and it worked.
“We hadn’t created anything from wide positions. Becchio and Ross McCormack needed the ball in the box and they got it in the second half.
“Luciano missed a good chance before he scored but he doesn’t freeze and went on to take his next chance so well.”
Jones revealed he expects top-flight clubs to come in for top-scorer Becchio and young defender Byram.
“If I was a Premier League manager I would be chasing them,” he said. “All the top clubs are watching Sam. They would be daft if they weren’t, but we have to keep those two and I think we will.”
Birmingham manager Lee Clark lost 19-year-old Will Packwood with a broken fibula and tibia, which will rule him out for the rest of the season.
Packwood and Becchio collided innocently soon after the equaliser.
“It took the gloss off a fantastic performance,” said Clark. “Will’s young team-mates and friends are distraught, they were in tears out there.
“I said, ‘Let’s try and win the game for Will’. We couldn’t quite do it but I thought we deserved to do so.
“We will give him as much care and support as possible and have him back fit and strong for next season.
“We should be celebrating a fantastic performance by my young players, but this is a real downer. I heard the snap and was hoping it was a shin pad. It was an innocuous one with two players going for the ball.”
Birmingham’s squad contained nine teenagers, four of them in the starting line-up. Yet it was 34-year-old Elliott who turned David Norris near half-way and steamed unchallenged through the middle to fire into the top right corner from 25 yards.
Norris atoned for his part in the opening goal by winning the ball off Curtis Davies and threading it through for Becchio to find the bottom corner for his 19th goal of the season.