Another source told ESPN's Adam Schefter that St. Brown could be out up to six weeks.
The sprain is probably the best-case scenario for St. Brown, considering he couldn't walk off the field without assistance Thursday night in Winnipeg, where turf conditions forced the NFL to play on an 80-yard field. The Packers pulled their starters shortly before kickoff.
St. Brown was in the top five on the depth chart behind Davante Adams, Marquez Valdes-Scantling, Geronimo Allison and Jake Kumerow, which means he almost certainly would have been in the rotation to start the season.
However, quarterback Aaron Rodgers called this year's receivers "one of the deeper groups that we've had." That's in part due to the return of Trevor Davis, who missed the first two preseason games because of a stinger, along with the emergence of Allen Lazard and undrafted rookie Darrius Shepherd.
Davis had more than 100 all-purpose yards against the Raiders, while Shepherd leads the Packers with two preseason touchdown catches. Lazard leads the team with 114 receiving yards on six catches.
Lazard also left Thursday's game against the Raiders after falling on his shoulder and head, but a source said he was expected to be available by the start of the regular season, if not sooner. The Packers play their preseason finale against the Kansas City Chiefs next Thursday.
Green Bay opens the regular season against the Chicago Bears on Sept. 5 in the NFL's annual Thursday night kickoff game.
If the Packers think Brown could miss six weeks or more, they could place him on injured reserve and bring him back later in the season. Teams are allowed to do that with two players.