Charlton, whose work ethic and team spirit have carried them to the unlikely heights of ninth in the Premiership, met their match in O'Neill's Leicester, who followed Saturday's 1-1 draw at League leaders Aston Villa with a thoroughly efficient second-half performance.
"We weren't at the races in the first half, but the manager gave us a rollicking at half-time and we were much more positive after that," the rejuvenated Cottee said, after scoring his third goal in two games.
Charlton were themselves on a high after their 4-2 win over West Ham and were indeed the better side in the first half. But although the good people of London SE7 had returned to The Valley in numbers hoping for a repeat of Saturday's extravaganza, it was not to be.
"Each Premiership game is going to be massive for us," Alan Curbishley, the Charlton manager, said. "We had a momentous game at the weekend and perhaps this one came around a bit too fast for us."
Curbishley had been forced into two changes, the injured Neil Redfearn and the cup-tied Carl Tiler making way for Keith Jones and the Welsh international John Robinson. Both were involved early on as Charlton searched for a quick opener.
First Jones slid a neat ball through for Clive Mendonca to chase but Kasey Keller raced from his line to block the striker's shot, then Robinson's sweetly timed effort from the edge of the area just cleared the bar.
Charlton's Australian goalkeeper, Andy Petterson, then reacted well to a Frank Sinclair header as Leicester retaliated but the clearest chance yet fell to Mendonca again after his fellow striker, Andy Hunt, had taken advantage of a slip by Matt Elliott. He played Mendonca in but a carefully flighted chip to the far post went wide with Keller well beaten.
Before the break the Charlton captain, Mark Kinsella, drew another smart save from Keller with a full-blooded drive, while Steve Guppy and Elliott had good headers well dealt with by Petterson. Neither side could be faulted for effort but the game was lacking a real competitive edge.
All that changed with three goals in the space of 15 minutes at the start of the second half. Muzzy Izzet found space in the inside-left channel and his cut-back from the byline bounced off Emile Heskey and Eddie Youds as they challenged for the ball. It fell kindly for Cottee, who made no mistake from 10 yards.
Four minutes later Charlton were level. Keller, attempting to punch clear above a crowd of players from Robinson's wickedly curled free-kick, could only direct the ball straight to Paul Mortimer, who dispatched it back past the helpless keeper.
Another four minutes and another goal, Cottee's second, after a neatly worked one-two with Izzet left the veteran forward with a simple tap-in.
Charlton kept trying but lacked inspiration and went close again four minutes into injury time when Sinclair raced across to block Mortimer's last-ditch effort. It was a close call, but by then Leicester had earned their place in the last 16.
Charlton Athletic (4-4-2): Petterson; Mills (Bright, 81), Rufus, Youds, Powell; Robinson (S Parker, 81), K Jones (Lisbie, 75), Kinsella, Mortimer; Hunt, Mendonca. Substitutes not used: Brown, Royce (gk).
Leicester City (3-5-2): Keller; Sinclair, Elliott, Ullathorne; Savage, Zagorakis, Lennon, Izzet, Guppy; Cottee, Heskey. Substitutes not used: G Parker, Walsh, Fenton, Wilson, Arphexad (gk).
Referee: G Willard (Worthing).Reuse content