Cobb, a 6-foot-5 shooting guard, will sit out this season and have two years of eligibility remaining.
"There are standards that each student-athlete is expected to uphold," Northwestern coach Bill Carmody said in a statement. "JerShon has our complete support as he works to fulfill those expectations and we look forward to having him rejoin the program in a full capacity in the future."
Cobb has started 33 games and has averaged 7.3 points and 22.5 minutes in his first two seasons. He was expected to provide the Wildcats depth in their backcourt this season as they attempt to reach their first NCAA tournament.