It was the same arm he broke in round one last season against Fremantle. The star Eagle missed just one game but was hampered for the rest of the year.
The already depleted Eagles will also be without Chris Masten (hamstring) for Saturday's crunch match against Geelong at Simonds Stadium.
Scans confirmed LeCras' forearm was fractured again and he was sent for surgery on Monday.
"He's really disappointed," teammate Dean Cox said.
"Hopefully he can get back now and run and touch the footy as much as he can and still have a big impact for this club because his pre-season was really good."
It is a cruel blow for the 2010 All Australian, who has had a luckless run with injury.
LeCras, 27, missed all of 2012 due to a knee reconstruction before managing 15 games last year. Following his broken arm in round one, he suffered a stress reaction in his foot late in the year and fractured his ribs in round 20.
"He's had a pretty bad run," Cox said.
"But Lecca is one of those players who will do everything right.
"He probably rushed it a little bit last year. So he will look into that and along with the medical staff. He'll come up with a plan for his program and when he will be back.
"He can play good football straight away."
Masten has also been ruled out for two to three weeks with a mild hamstring strain as the result of a bad cork he received during the win over the Saints.
The Eagles have already lost Shannon Hurn for at least a month with a knee injury.
They also may be without key midfielder Luke Shuey for a week after the Match Review Panel charged Shuey with striking St Kilda's Tom Curren last Saturday night.
Shuey can accept a one-week ban with an early guilty plea but it would see him miss the clash with the undefeated Cats.
The South Fremantle great, a legend of WA football and widely regarded as the best player to never play in Victoria, has become just as celebrated for his contributions off the field after his playing career
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