Whitecross' right knee buckled in a marking contest just minutes into Friday night's qualifying final against Collingwood and he was duly substituted out of the game.
Hawks football manager Mark Evans confirmed the worst on SEN on Saturday morning.
"It was as suspected, the doctors knew straight away there was something wrong with that knee, there was too much movement," Evans said.
"We can confirm that he (Whitecross) has ruptured his anterior cruciate ligament."
Evans said it was shattering news for Whitecross and the club.
"It's bad news for him. He's a good footballer who's come a long way, but more than that he's a terrific person."
But Evans praised Whitecross' positive attitude through it all, as he was seen smiling and encouraging teammates on the bench.
"He was determined to have a really positive demeanour and a really positive impact on players who were coming to the bench," Evans said.
"All that from a guy who was told that he's quite likely done some serious damage to his knee is just amazing."
Jordan Lewis and Clinton Young were notable absentees from the Hawks' line-up, but Evans said the pair should be right to take their place in the preliminary final team.
Lewis had "tightness" in his hamstring and Evans said the Hawks decided to take a cautious approach and not risk losing their star on-baller to a possible tear.
Evans said Friday night's game had taken a toll on the players and that the rest week would be extremely beneficial before another tough preliminary final.
"We're pretty pleased to get a breather this week and then get set for the next one," he said.
Ben Guthrie is a reporter for AFL.com.au. Follow him on Twitter- @AFL_BenGuthrie
