Hawthorn youngster Beau Muston will require knee surgery for the third time and have his long-delayed AFL debut set back by another season.

Scans confirmed the highly-rated 21-year-old tore the anterior cruciate graft in his left knee in Saturday night's pre-season semi-final loss to Adelaide at AAMI Stadium.

Muston was drafted by the Hawks in 2005 despite coming off a knee reconstruction and needed further surgery after an examination showed the graft and screws had loosened.

That meant he could not play at all in 2006.

He played some VFL football in 2007 and was performing well in the current pre-season, before Saturday night's injury, which occurred on his 21st birthday.

Muston's manager Paul Connors said the promising midfielder had been all but assured of making his AFL debut in the Hawks' opening round match against Melbourne.

"He woke up Saturday morning and life was pretty good, he was 99 per cent sure of playing his first ever AFL game in round one against the Dees ... it's his 21st birthday and life's great," Connors told Melbourne sports radio station SEN.

"A few hours later it's turned around."

Connors said there had been little he could say to comfort Muston, despite pointing out other players, such as Richmond's Mark Coughlan and Carlton's Richard Hadley, who were playing after multiple reconstructions.

"As he said, (those recurrences) happened straight away, this has been a year-and-a-half of a full pre-season, a whole season of playing, another pre-season," he said.

Connors said it would be a tough road back emotionally.

"The players' association are a wonderful organisation, let's not take away from Hawthorn, it's a great club and he'll be surrounded by good people," he said.

"But there's going to be some ups and downs for Beau along the year I'm sure."

The Hawks said they would continue to support Muston during his rehabilitation.

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