The Cubs have had an odd recent history of developing SPs who have middling profiles while coming up through the farm, and then seeing them break through those perceived ceilings. Assad and Steele are both recent examples, but you can also point to guys like Alec Mills, Kyle Hendricks, and even Jake Arrieta among that group. It's not a constant stream of guys, but it's enough to make you think that it will happen again.
It's why Birdsell is intriguing to me. He's not exactly a stuff monster, but he offers a really different look from other pitchers and there's enough there to make you think he could develop enough of an arsenal to handle lefties regularly and go deep into games.
He should get a look next season once the usual injury attrition starts to hit the rotation, and I'm curious to see how he adjusts once the book gets out on him. To be clear, I'm not expecting an ace out of him, but it would be nice to find a mid-rotation anchor for the next few years.