Marlon King broke 10-man Millwall's hearts with a late winner to put a severe dent in The Lions' Play-Off ambitions.
King had broken the deadlock on eight minutes only for his strike to be cancelled out by a superb Andros Townsend equaliser on the stroke of half-time.
But then Neil Harris, who had only been on the field a matter a seconds after replacing Jimmy Abdou at half-time, was shown a straight red card by referee Roger East for an off the ball incident with Sky Blues Aron Gunnarsson.
Chopper's dismissal left The Lions a second-half mountain to climb and despite a tremendous effort from his 10 remaining team-mates, former Gillingham striker King - so often Millwall's nemesis - slammed home from close range with just four minutes to go.
It was a bitter blow for Kenny Jackett's side and compounded an afternoon that could prove costly in the remaining four games.
Not only does Chopper now join fellow frontman Steve Morison on the sidelines for three games, but skipper Paul Robinson was also ruled out just before kick-off.
Jackett was forced to reshuffle his playing pack again with Jimmy Abdou coming into midfield and Liam Trotter moving further forward to support striker Josh McQuoid.
Scott Barron also came back into the fray at the expense of Tony Craig, who dropped to the bench, but then Jackett had to re-jig his defence late on with Robbo dropping out after the teams had been handed in.
Darren Purse came in alongside Darren Ward with Alan Dunne taking the captain's armband.
Unfortunately with just eight minutes on the clock Purse horribly underhit a square pass to Dunne, King nipped in, rounded keeper David Forde and stroked the ball into the bottom right hand corner to hand the Sky Blues a 1-0 lead.
It took The Lions a while to recover not only from the early setback, but also to get the measure of their hosts.
As the half wore on, so did the influence of wide men James Henry and Andros Townsend with the former in particular at times appearing to be on a one man mission to fashion an equaliser.
On 35 minutes Henry picked up a sliderule pass from Dunne, powered forward leaving a handful of City players trailing in his wake but could not provide a finish to match his splendid run, sending a left-foot shot that flew comfortably wide.
Two minutes later Henry opted to go it alone and instead of spraying the ball wide to overlapping right-back Dunne continued forward. On this occasion his stinging shot was blocked and the danger averted.
A slick move culminated in Townsend and Henry exchanging passes before Henry's arrowed drive flashed wide.
However, when Millwall finally did grab the equaliser their pressure threatened, it was no surprise that the Townsend/Henry combination came up trumps on the stroke of half-time.
Henry intercepted a poor crossfield pass from King, drove on with power and purpose yet again and following some neat interplay with Townsend, eventually released the ball to his fellow wide man 20 yards out. Townsend took a couple of touches before firing an unstoppable shot into the bottom left corner giving keeper Keiren Westwood no chance.
With the game finely balanced, Lions boss Jackett brought on Harris for Abdou with the intention of operating a 4-4-2 system.
But for all the planning, all the tactical talking during at half-time, that plan was sent flying out of the window with less than one minute of the second-half gone.
Harris and Gunnarsson tangled for the ball and although play continued, Gunarsson went to ground behind the referee's back and on the advice of his assistant, a red card was brandished to the Millwall man, who sprinted off straight down the tunnel with barely a glance to the visiting bench.
The red card proved to be the turning point. It was going to be an uphill struggle thereafter, but immense credit should go to the remaining 10 lionhearts who produced as gutsy, intelligent and forceful performance as possible in the circumstances.
Admittedly it took a breathtaking save by keeper Forde to tip Carl Baker's piledriver onto the woodwork and over for a corner on the hour mark, but The Lions were a real threat themselves on the counter-attack.
McQuoid nearly bagged his first goal in a Millwall shirt after 63 minutes with a jinking run into the area, but could not find the target with his shot.
The tireless Trotter then won possession in midfield, spread the ball wide to the lively Henry and his delicious delivery was tipped away by Westwood at full stretch.
As the game headed towards the final five minutes it looked as if The Lions gargantuan effort was going to pay off in the shape of at least one point.
But with Trotter lying on the ground after falling awkwardly in another Millwall attack, Coventry broke forward and made full use of their two-man advantage, with King providing a clinical close range finish to put the Sky Blues 2-1 ahead.
Even then there was still time for Tam Mkandawire's drive to be hacked off the line in a frenetic finale that also saw King denied a hat-trick first by the post, which kept out his curling effort, and then by a brilliant one-handed save from a flying Forde at full stretch.
The final whistle may well leave only the most optimistic of fans believing that a top six finish is still up for grabs, but there are 12 huge points to play for - and the Fat Lady ain't singing just yet!
Coventry City: Westwood, Keogh, Clingan, Bell, King, Jutkiewicz (Platt , 65mins) Hussey, Cranie, Gunnarsson (McIndoe, 51mins), Baker, Clarke (Wood, 77mins). Subs not used: Quirke, O'Halloran, Eastwood, Deegan.
Booking: Jutkiewicz (81mins)
Scorer: King (8, 84mins)
Millwall: Forde, Dunne (Marquis,90+2mins), Barron, Abdou (Harris, 46mins), Purse*, Ward, Townsend, Mkandawire, Trotter, McQuoid (Bouazza, 70mins), Henry. Subs not used: Mildenhall, Craig, Hackett.
Sent off: Harris (46mins)
Scorer: Townsend (45mins)
* Late replacement for Robinson
Ref: R. East
Att: 16,3454 (away 1,109)