LUTON, England (AFP) - Roy Keane insisted he had warned his Sunderland players not to underestimate Luton before the League One club knocked the Black Cats out of the League Cup.

Keane feared complacency might be a problem because Sunderland had thrashed Luton 5-0 at the end of last season to clinch the Championship title.

The former Manchester United star was proved correct as Luton exacted revenge with a superb 3-0 second round win over the Premier League newboys on Tuesday.

Keane made only four changes to the team beaten by Liverpool on Saturday, but, although his side looked strong on paper, they were no match for Luton on the pitch.

David Bell opened the scoring with a blistering free-kick in the 16th minute and veteran striker Paul Furlong added a second when he lobbed goalkeeper Darren Ward just before half-time.

Sunderland's misery was complete after the break. Greg Halford was sent off for a second booking and Furlong put the icing on the cake for Luton with a close range strike in the 75th minute.

"We discussed it before the match and we knew it was going to be a lot more difficult than last year when we came here riding high," Keane said.

"We thought the team we put out would be strong enough to win the game but it wasn't to be."

It was a bad day all around for Keane, who found out earlier on Tuesday that influential midfielder and captain Dean Whitehead will be out for six months with a cruciate knee ligament injury.

"It looks as though Deano will need surgery in a couple of weeks," Keane said. "It's bitter blow. We are missing his leadership qualities and his presence."

The only silver-lining for Keane is the potential signings of Stoke defender Danny Higginbotham and Southampton striker Kenwyne Jones.

The Irish coach has been granted permission to talk to both players by their respective clubs and hopes to complete the deals before the transfer window closes on Friday.