The in-form small forward hurt his left elbow again in Saturday night's loss to Fremantle.
Scans on Monday dashed any hopes of good news and he will undergo surgery this week.
Weightman has been playing with strapping around the elbow after dislocating it two years ago, then suffering the same injury in round three against West Coast.
His arm was in a sling after the Dockers loss and coach Luke Beveridge said they might have to take a long-term approach after this latest setback.
The Dockers loss leaves the Bulldogs in trouble at 3-4, with Rhylee West also out through suspension for Sunday's match against Hawthorn.
There is some good news for the Dogs, with star forward Jamarra Ugle-Hagan expected to return after two games out on personal leave.
Top midfielder Tom Liberatore has also been sidelined, firstly because of his worrying collapse in the round-six loss to Essendon.
He was also a late withdrawal from the Fremantle game with illness, but will return to play the Hawks.
Also on Monday, the Hawks confirmed forward Sam Butler will miss the rest of the season after needing surgery on a broken leg.
The 21-year-old was hurt on the weekend playing for their Box Hill VFL affiliate.
Geelong are also yet to confirm the severity of Patrick Dangerfield's latest hamstring problem.
The Cats captain, who has a history of hamstring problems, was injured again in Saturday's win over Carlton.
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