The tendinitis was aggravated following a knee-on-knee collision in Boston's summer league opener against Philadelphia on Friday.
Williams left that game after just six minutes of action, tallying four points on 2-for-4 shooting along with two rebounds.
"Tendinitis is something terrible, but I'm looking to come back as soon as possible," said Williams who added that he's taking it "day-to-day."
His absence from the last two summer league games only adds to what has been a disappointing start for Williams who was selected with the 27th overall pick by the Celtics in last month's NBA draft.
Williams said he first began having tendinitis-related issues this past season at Texas A&M.
While it's not considered something that will sideline him for a significant amount of time, it is something that the Celtics know they have to stay on top of to ensure that it doesn't become a major issue in the future.
For Williams, not being able to play has been somewhat frustrating for him.
"It's very frustrating. Everyone is out here, everyone is watching," Williams said. "You just have to put that in the hours of work. Have to support my teammates, help the team any way possible."