Bulls head coach Jim Boylen provided further details, per Woodyard:
"It's a soft tissue injury so it's something that's difficult to put a timetable on. That's about as easy as I could say. We're not exactly sure.
"I don't want to give you a definite timetable because it could be sooner or longer than maybe we see. I know that's vague and I'm sorry for that, but that's the kind of injury it is."
It's already been an unlucky season for Porter for injuries with several different issues holding him back.
This came after a shoulder injury cut his 2018-19 season short after just 56 games, including 15 with the Bulls after a midseason trade.
Though few of his injuries caused him to miss time, the constant problems have prevented Porter from reaching his full potential with Chicago.
The wing proved to be a great all-around player during his time with the Wizards and stepped up offensively last season in Chicago with 17.5 points per game while knocking down 48.8 percent of shots from three-point range.
So far in 2019-20, Porter is averaging 11.2 points and 3.4 rebounds per game.
The Bulls could utilize some smaller lineups if Porter is forced to miss time, while Chandler Hutchison could also see more action at small forward.