The damning truth is that as long as Mitchell is healthy, Frelick just isn’t a great fit to be on the MLB roster right now. He doesn’t have a skill set that plays well off the bench. Mitchell is better in just about every regard as a starter.
So you’re left with a starter who doesn’t do anything particularly well and even with the matchup he wasn’t a great option off the bench in the 8th.
With Lara signed long-term and no service time considerations anymore, there’s nothing left other than MLB track record to continue with Frelick up over Lara, and even that argument is losing steam quickly.