Burnley, tipped by many to struggle at the wrong end of the table this season, took full advantage of the sending off by racing into a stunning 3-0 lead courtesy of two goals from Sam Vokes and another from Stephen Ward.
Chelsea pulled one back midway through the second half as record summer signing Alvaro Morata came off the bench to head home but any sign of a comeback vanished when Cesc Fabregas was dismissed for a second booking in the 81st minute.
Burnley produced the first shock result of the Premier League season, beating champions Chelsea with a remarkable performance at Stamford Bridge.
And with just over 20 minutes remaining, Premier League debutant Morata - who had replaced the inauspicious Michy Batshuayi - met an excellent Willian cross and guided a header into the far corner.
The injury luck that afforded him the most settled XI in the division last season abandoned the Blues weeks ago and their lack of squad depth was achingly apparent against Burnley. Boga, an exciting talent, had little time to make any sort of impact before becoming the unfortunate collateral damage of Cahill's early red, sacrificed for Andreas Christensen in the 18th minute.
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Chelsea were already without the likes of Eden Hazard, Victor Moses and Pedro as they began their title defence at home to Burnley on Saturday afternoon.
Mauricio Pellegrino had to settle for a point in his first match as Southampton manager as his side wasted a series of chancs. David Luiz scored a late consolation goal for the defending champions. Vokes had a goal disallowed shortly after - but it was an omen for Chelsea.
New-look Everton had marginally the better of a disjointed opening spell with Sandro, another of their new signings, seeing an effort trundle just wide of the Stoke goal.
That didn't stop the Blues from making it 3-2, as Morata flicked onto Luiz for a cool finish.(NBC Sports).