Attwood, who will join Bath from Gloucester ahead of the new Premiership season, suffered the injury in the Saxons' impressive 41-14 win over Tonga at Kingsholm on Sunday.
Sale back-rower Carl Fearns has been called into the squad as a replacement, with his Sharks club-mate James Gaskell set to provide second-row cover.
An RFU statement read: "Carl Fearns has been called into the England Saxons squad as an injury replacement for Dave Attwood ahead of this weekend's Churchill Cup final against Canada.
"Lock Attwood has been ruled out of the game by a right knee injury sustained in the Saxons' 41-14 win against Tonga on Sunday and is due to see a specialist tonight."
The Saxons had already lost captain Luke Narraway to a back injury after the Gloucester number eight complained of soreness before the tournament opener against the USA.
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