The Tigers had seemed on course for a draw in a rain-lashed engage Super League thriller at The Jungle when Shenton conceded a late penalty.
There were just five seconds remaining when Shenton was penalised for offside in front of the posts after Brent Sherwin had lost the ball.
Leeds skipper Kevin Sinfield made no mistake and slotted the ball between the posts for a dramatic 24-22 win.
Matterson said: "He's devastated. He is a local lad and is gutted, he has grown up watching Leeds-Castleford derbies.
"But he had no choice. If he doesn't dive on it Leeds pick it up and score."
Shenton had levelled the scores with his side's fourth try just minutes earlier.
In a match played in a torrential rain, Leeds had raced ahead with early tries from Rob Burrow, Danny McGuire and Ryan Hall.
Cas hit back to level at 16-16 through Stuart Jones, Kirk Dixon and Rangi Chase with Shenton touching down after McGuire's second had put Leeds back ahead.
Matterson added: "It can be a cruel game at times. Probably 22-22 would have been a fair indication of the game.
"For large parts we were on top and I thought it was a cracking game of rugby.
"For both teams to play the rugby they did, it was pleasing on the eye.
"I don't think we deserved that but if we keep putting ourselves in that position we will get the bounce of the ball."


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