Lowry, 30, is having the best season of his 11-year NBA career, averaging 22.8 points, 6.9 assists and 4.7 rebounds.
The Raptors are 35-24 and tied with the Wizards for third place in the Eastern Conference standings. They are only 2.5 games ahead of the fifth-place Atlanta Hawks, so losing Lowry for what looks like at least the rest of the regular season could cost Toronto home-court advantage in the first round of the postseason.
Last year, the Raptors finished second in the East and made it all the way to the conference finals, where they lost to the eventual NBA champion Cleveland Cavaliers. Their path to a repeat of that performance will be a whole lot tougher without Lowry, who plays a team-high 37.7 minutes a game.