Awful, awful news for the lad.
It brings back how I felt at the time about this, and also the Mane high boot (Which was obviously a lot, lot less serious) and how we look at challenges in the air differently from those on the deck.
If you mis-time a tackle and catch someone halfway up the shin they show it frame by frame and discuss how it's a definite red and how it could be a career ending injury or a leg breaker.
If the ball is in the air and you mis time it and head-butt someone or kick someone in the head we hear, "ah it was an honest effort to play the ball" or " the ball was there to be won" or even "he had every right to go for that", as if you have no right to put in a sliding tackle.
Players rarely go into a tackle to hurt an opponent, but if you go in with excessive force whether it's a sliding tackle, a header or a high boot, and you put an opponent in danger like that, then it's a definite red card.
Cahill, I'm sure, had no intention of hurting Mason. But he should have seen red, for what turned out to be, an actual career-ender.