Teenage substitute Keane Lewis-Potter deprived the hosts at the death with a header that squirmed in off Charlton goalkeeper Dillon Phillips for an own goal.
Darren Pratley put the home side in front before Jarrod Bowen’s equaliser just after half-time was followed by a close-range finish from Naby Sarr.
Lee Bowyer’s side looked set to hang on for three points and end a nine-match winless run, but that opportunity was snatched away in the most agonising circumstances.
But McCann admitted his team were very much wecond-best in the opening 45 minutes.
“We were poor first half. We couldn’t pass a ball to an amber player. Me stood on the touchline with Cliffy (assistant Cliff Byrne), we didn’t know what to say to each other.
“And the second half, we controlled it. To get an early goal was pleasing.
“To concede so quickly after was shocking really because we pride ourselves on counter-pressing, reactions.
“We just let Naby Sarr run the full length of the pitch without anyone tracking him and our reaction was poor, but what the group has is a real steel to keep going. I’m delighted and relieved. We’ve got out of jail.”
McCann reserved special praise for Lewis-Potter, who came on in the second half to make a decisive impact.
He added: “He’s not the biggest, but he’s leapt against Barnsley to get us back in the game and again today, against big centre-backs, so I’m really, really pleased for Keano.
“He’s at the right place to keep developing and keep his feet on the ground. He can be what he wants. If he keeps doing that, he’ll push himself closer to starting for the first team.
“If you’re going to give that as an own goal, then any shot that anyone hits now and the keeper touches, is that an own goal? I don’t know.
“I’m sure Dillon Phillips doesn’t want it, so give it to Keane.”