Far more important than who replaces Butler is the fact that the Bulls now have a timetable for how long they could be without their leading scorer. Whereas this was once Derrick Rose's team, Butler is now the unquestioned leader.
He's also one of the best 2-guards in the league, per ESPN Stats & Info:
ESPN Stats & Info @ESPNStatsInfo
Jimmy Butler out 3-4 wks (incl. ASG), strained knee. Butler: second (+4.56) among SGs in Real Plus-Minus to James Harden (+4.74)2/9/2016, 6:53:47 PM
Chicago is only a game-and-a-half ahead of the Charlotte Hornets, who are ninth in the Eastern Conference, and only four games separate the Bulls from the Orlando Magic and Washington Wizards, the 10th and 11th seeds.
NBA Commissioner Adam Silver doesn't have an enviable task ahead as he fills out the East roster for the All-Star Game, but Bulls head coach Fred Hoiberg is in a much tougher position as he looks ahead to the next month with his best player out of action.