The forward has struggled to make an impact for the Scottish Championship leaders this season.
The 24-year-old has struggled to hold down a place in Mark Warburton's side this season, and began the game against the Scottish Premiership side on the bench. However, an injury to leading scorer Martyn Waghorn meant that Clark was brought on after just seven minutes of the clash at Rugby Park.
And he showed exactly why he should be considered as Waghorn's replacement if the former Wigan man is set for a spell on the sidelines, as he bagged the late winner in the 2-1 victory to send the Light Blues into the quarter-final. And McKay explained that Clark's goal sent the perfect message to Warburton.
"It's been hard for Nicky, he's done well when he's played and he's been scoring but it's been difficult for him with Kenny [Miller] and Waggy ahead of him," he said, as reported by Sky Sports.
"But tonight he has taken his chance with both hands by getting us through and he's certainly staked a claim for a regular start. It's a blow for Waggy to be injured but it's a chance for Nicky to get himself in there and keep himself in there."
While Waghorn's tally of 20 goals in 24 league games means that he should surely come back into the side once he has proved his fitness, Clark's winner will arguably show to Warburton that he can be confident that if he needs to turn to the former Queen of the South striker in the future, he can deliver, especially as the goal came against top flight opposition.
And being able to have faith in a cluster of strikers, rather than put the majority of the pressure on one forward, will surely only help the Gers as they look to go far in the Scottish Cup while also maintaining their position at the top of the Championship.