Rosa underwent a procedure to release pressure from potentially dangerous compartment syndrome, which he suffered during the Eagles' six-point NAB Cup loss to Port Adelaide on Saturday in Alice Springs.
The latest injury setback for Rosa will rule him out for a month and a half, according to West Coast coach John Worsfold.
“Matt Rosa pulled up sore after the Port Adelaide game, our medical staff assessed him and unfortunately he has compartment syndrome which required surgery," Worsfold revealed to the West Coast website.
With the Eagles' blockbuster Round 1 clash against cross-town rivals Fremantle a little over a week away, it is a major blow for a side which will be without several other key stars.
Cult figure ruckman Nic Naitanui won't be available for the first month of the season, after undergoing groin surgery over the summer.
The Eagles say they expect Naitanui to re-join the main training group sometime late this week.
And on-ball superstar Daniel Kerr is battling knee problems and the club has declared the ball-winning veteran will not be rushed back into action.
“We were hopeful that Nic would recover in time to play, but we were never going to rush him back into the team,” Worsfold continued.
“It will be good to have him in our main training group this week after spending so long in rehab and will slowly build up his work load so that he has the appropriate level of conditioning to play when that time comes.
“Daniel has also been working hard, but needs more time. Again we will not rush him and there is no timeline for his return. When he is right to play, he will play."
Star recruit Sharrod Wellingham is also still in recovery from an ankle problem and will also miss the derby opener.
But, Worsfold revealed Norm Smith medallist Andrew Embley, who has been battling hamstring tightness, would be available for the season opener on March 23 at Patersons Stadium.
“It is disappointing for those players, but it creates opportunities for others," Worsfold said.
"Andrew Embley is right to go, Chris Masten, who was a late withdrawal against Port, should be available and we have had a number of players in good form in WAFL reserves matches in the last few weeks.
“There is pressure on for places in the team and all available players are aware of that.”