Tom Coventry tells BBC Radio Berkshire a Halani Aulika sin-bin led to London Irish "capitulating" in their 37-5 loss to Northampton.
The Tongan prop was given a yellow card after 15 minutes, a moment Coventry described as "the turning point in the game".
"They scored a couple of tries from the back of that and grew in confidence. They finished pretty strongly after that," said the 49-year-old.
The result means they return to the bottom after the Premiership, after Worcester picked up a bonus point against Bath.