The 21-year-old midfielder has played eight games in three seasons, including no senior matches in an injury-interrupted 2013.
Browne ruptured his anterior cruciate ligament during Monday night's pre-season game against Gold Coast at Metricon Stadium.
"Alex was very unlucky," club doctor Bruce Reid said on Essendon's website.
"His leg got caught under his body on the field and as a result he has torn his cruciate and medial ligament.
"Unfortunately he does need a full, traditional reconstruction."
Essendon coach Mark Thompson said Browne had been enjoying his best pre-season campaign.
"While it's a long road to recovery for players who require reconstructions, we have no doubt that Alex will come back stronger," Thompson said.
Ruckman Tom Bellchambers (ankle) is also on the sidelines and is expected to miss the first month of the season. Essendon's first game is against North Melbourne at Etihad Stadium on March 21.