The highly rated Bendtner is now facing the prospect of being out of the side for several weeks which will be a huge blow to the Midlands club.
But the Championship leaders, reduced to only ten players for the entire second half, battled back against all the odds against Newcastle to earn a second chance.
Injury-hit Newcastle failed to take full advantage of the situation and often allowed Birmingham to dictate most of the play in the first half when Dudley Campbell gave them a 15th minute lead.
But the game turned on its head in a dramatic couple of minutes just before half-time when Jaidi had a rush of blood to the head. The veteran central defender was guilty of ball watching as he allowed Steve Taylor to equalise in the 40th minute.
A couple of minutes later Jaidi brought down Obafemi Martins as the Newcastle striker was heading goalwards. Raidi was immediately shown the red card and from that moment Birmingham were put on the back foot.
Their prospects looked far from rosy early in the second half when James Milner, who had a very quiet game, lashed a free-kick against the angle of the bar and upright. The ball crashed down, hit goalkeeper Maik Taylor, and bounced away for a corner.
United were encouraged by this incident and produced their best spell of the game with Nicky Butt emerging as their key player in midfield and they appeared to be heading into the next round when Kieron Dyer put them ahead.
It was another woeful mistake which led to the goal. Martins produced an innocuous throughball which went under the feet of the lumbering Martin Taylor. Dyer, rapidly regaining some of his best form following a lengthy spell out of action, took the ball on before beating Maik Taylor in the Birmingham goal.
The game appeared to be cut and dried with the Magpies apparently easing to another cup win.
Birmingham's hopes of salvaging the game seemingly disappeared when Bendtner damaged his ankle twisting on the muddy, rain-soaked pitch.
The youngster had previously given Paul Huntington a harrowing time and his departure could have a dramatic effect on Birmingham's promotion chances.
Birmingham themselves are to rip-up their pitch on Sunday following numerous complaints and relay it at a cost of £120,000 but this is of little comfort to Bendtner.
The Dane's departure, however, seemingly lifted Birmingham and they launched another of their now well-known late finishes to salvage a draw.
Skipper Damien Johnson, who produced a steady performance, set up the face-saving raid with a cross from the left which found Sebastian Larsson.
The on-loan winger, who had earlier in the game occupied a defensive position following the enforced departure of Stephen Kelly, collected the ball and shot on the turn to beat Shay Given five minutes from time.
It was a well-deserved equaliser for Birmingham but Newcastle can only blame themselves for giving away such a late equaliser.
They survived a bad start to the game when Huntington and Solano were often caught napping.
It was the inspired Stev Taylor who lifted them out of the doldrums. Birmingham were flying when Campbell turned home a Matthew Upson header in the 15th minute and it was only a goal-line clearance by Taylor to prevent Fabrice Muamba scoring, that kept United in the game.
Taylor then went up for a Milner corner and, with Jaidi ball-watching, stabbed his shot home for the equaliser.
Jaidi then got himself sent off to seemingly hand United a win on the plate, but they reckoned without Birmingham's fighting spirit which earned them a replay albeit at a heavy cost.