If it seems like every time that the Celtics have a chance to win it all, someone very major gets hurt, it's because it's 100% true. 2009 there was KG. Then in 2010, KG was still not back to 100% and Boston looked liked they would win regardless and then Perkins went down. Then in 2011 Rondo. 2012 Jeff Green out for the year. Then Rondo torn ACL in 2013.
Flash forward to when the Celtics got good again and Isaiah Thomas goes down with his hip issue, then Hayward and Kyrie, and on and on it goes. Celtics could have won the Micky Mouse bubble title, but Hayward got hurt again and Kemba's knee... You get the point.
We haven't heard a timetable for Williams' return, but usually they repair meniscus tears with young players in a way that's best suited for their long term health, which also tends to knock them out for the year, so brace yourselves.
This is why every season, I tell people to stop crying so much about standings. That the most important thing for the Celtics is for them to be healthy. A regular season loss is never devastating. You never hit rock bottom due to just a bad loss. Major injuries, especially to your top guys, is the gut punch.
The Green Teamers will say that the Celtics will still win the championship this season, but objective fans know that their odds will go way down unless Time Lord is able to return at 100% for the playoffs. Yes, Boston can still make a lot of noise with Theis stepping up for Williams, considering how good the rest of the team has been playing, but this team had a depth issue and now they really have one. They will need every other rotation player to be healthy for all their playoff games. That's a tall task. And since the Len Bias death, the Celtics luck has sucked.