Indeed, Siemian started in Fields' place despite injuring his oblique during warmups. He was ultimately able to play through the injury. However, Adam Schefter reported that Bears' HC Matt Eberflus told reporters that QB Trevor Siemian will undergo season-ending surgery to repair the oblique injury he suffered during pregame warmups last Sunday.
This means that, if Justin Fields is to miss more time, it will most likely be Nathan Peterman under center for the Bears offense.
However, we expect Fields to be back in the starting lineup this weekend against the Green Bay Packers. Fields does not have an injury designation for Sunday's game against the Packers. He missed last Sunday's loss to the Jets with a left shoulder injury, but moved back to full practice participation on Thursday.
Peterman will now be joined by Tim Boyle as backup options behind Fields.