DESPITE having already lost his first pick from last year's draft to a season-ending knee injury, Melbourne coach Paul Roos says practice matches and NAB Challenge games are essential preparation for round one.

No.2 draft pick Christian Petracca hurt his ACL at training a fortnight ago, while West Coast's Eric McKenzie and the Western Bulldogs' Tom Liberatore both suffered season-ending knee injuries during NAB Challenge games at the weekend.

The cruel blows led to discussions about the importance and risks of pre-season matches and players’ training loads.

"You don't want to see any players get injured but that is just the reality of the game unfortunately," Roos said.

He said he was happy with the NAB Challenge format of three games with an extended interchange bench for each club’s opening two matches.

Roos said players could not go straight into round one without practice matches.

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"[I] just don't think you can go from doing nothing to playing a game. [The] jump would be just too dramatic, the injury rate would be enormous and the standard of footy would be pretty ordinary," Roos said.

After having an operation to repair his ACL last week, Petracca was still in good spirits despite knowing he would be on the sidelines for the remainder of 2015, Roos said.

The 18-year-old had even floated the idea of getting back before the season ended, but the experienced Roos quickly knocked that exuberant idea on the head.

"The first thing he (Petracca) said to me [was] 'oh the doc said I can play in four or five months', and I said 'well the coach said you can't'."

Despite the obvious disappointment Roos said the club would work hard to turn Petracca's absence into a positive.

"[We'll] try to keep his mind active," Roos said.

"What we've learned from Jesse [Hogan] is they can really improve in different areas if they do have a long term injury," Roos said. "We'll get him back training as soon as we can, but there will be no thoughts of him playing this year."

The coach knows such things happen and, like coaches and supporters of all clubs, Roos will keep his fingers crossed his club doesn't cop more than their fair share of such luck.

"You hate to see guys get injured. No-one likes to see it but unfortunately that is the industry we're in," Roos said.