Only when Northampton's Ian Clarkson left stud marks on the midfielder Muzzy Izzet's thigh 20 minutes from the end did O'Neill, who is facing a misconduct charge following a verbal attack on a match official last month, stir from his seat in the dug-out.
The referee Alan Wilkie instantly dismissed the Northampton player, whose sending-off was quite out of character with a match lacking real passion.
"It was a very professional performance by Leicester but a lot of my players did not do themselves justice, which was disappointing," Northampton's manager, Ian Atkins, said. Clarkson, he added, would be fined a week's wages if the video confirms he stamped on Izzet.
Northampton brought a side laden with experience, despite a collective assembly value of just pounds 125,000. They included Jason Dozzell, the former England Under-21 international for whom Tottenham once paid pounds 1.9m.
O'Neill admitted to having been "worried" by the nature of the tie after a sticky patch in the Premiership but Northampton were never in contention once Leicester had their measure. The home side were soon in control, sweeping into a two-goal lead by the 26th minute.
They enjoyed some good fortune. The first goal, swept home by Ian Marshall, came after Emile Heskey had inadvertently trodden on the ball in attempting to round the goalkeeper Andy Woodhead, a turn of events that left both Woodhead and the defender Charlie Bishop off balance and powerless to intervene as Marshall pounced.
The second, scored by Garry Parker, came from a penalty after John Frain had tripped Robbie Savage, who was one of Leicester's outstanding performers. Savage weighed in with the third and best goal early in the second half, after which that seasoned campaigner Tony Cottee converted Heskey's low cross to record his first goal in a Leicester shirt.Reuse content