Pritchard left Friday's 122-110 loss to the Philadelphia 76ers in the the first quarter after teammate Jaylen Brown fell into his leg. The Celtics called the injury a "right knee sprain" at the time, and further tests revealed a Grade 1 sprain of the MCL.
The former Oregon star has made an immediate impact in the Celtics lineup as a reliable ball-handler. Through his first 14 games, the 22-year-old is averaging 7.7 points, 2.6 assists and 2.4 rebounds while shooting 48.8 percent from the field in 20.9 minutes per night.
Kemba Walker's recent return from his own knee injury lessens the blow, and look for head coach Brad Stevens to rely more heavily on the likes of Marcus Smart, Jeff Teague and Javonte Green until Pritchard returns.
In the meantime, the injury adds another issue for Stevens and the Celtics to work through.
Boston is on a three-game losing streak and hosts a surprisingly tough Cleveland Cavaliers team Sunday. At 8-6, the Celtics remain second in the Atlantic Division but could see their standing slip if they can't correct course soon.