5. Jordan Montgomery
It kind of feels weird to think about bringing back a member of the rotation from 2023 as an upgrade, but in all honesty, Jordan Montgomery would be a huge get as the second or third starter in this rotation.
Montgomery was pitching extremely well for St. Louis before being traded, and since arriving in Texas, he's posted a 2.79 ERA and already balled out in his first postseason game against the Rays. He's proving himself to be a big-game pitcher to all of the would-be suitors this offseason.
John Mozeliak has not closed the door on a reunion with Montgomery, and although I think they will prioritize other names, there are definitely scenarios where Montgomery is one of their guys. He's likely earning himself a long-term in the $25 million a year range, so he will not come cheap, but St. Louis also gets the advantage of familiarity, which may be appealing to them. Bringing in so many new pitchers may create some challenges for the coaching staff and catchers when it comes to building chemistry, so Montgomery would give them a leg up in that regard.
It would be a big miss from the front office if Montgomery was the best addition they make, and I think they know that as well. But getting a guy who can eat innings and do it at a high level is something this rotation needs badly, and Montgomery fits that bill.