Reed Johnson and Paul Maholm were dealt to the Braves on Monday night, Geovany Soto was sent to the Rangers early Tuesday and Ryan Dempster was traded to Texas just prior to the non-waiver Trade Deadline.
Soto had started 51 games compared to 28 by Steve Clevenger, and Cubs manager Dale Sveum said Clevenger and Castillo will share the job.
"Probably right now, just splitting time," Sveum said. "I think it'll be a little more 50-50 now."
Coleman (0-1, 7.32 ERA) started Tuesday night in place of Dempster.
Sveum expected to meet with general manager Jed Hoyer and discuss the roster on Thursday's off-day.
"We'll probably sit down in the next few days and sit back and say, 'OK, what do we want to do in the last 50 games of the year, the last 60 games of the year,'" Sveum said.