The Tigers could get by with the five starters they already have, but it's been reassuring to fans that they're actively looking at some reinforcements this offseason. They were interested in converting Ryan Helsley into a starter before he signed a two-year contract to remain a reliever with the Orioles, and they're reportedly interested in successful reliever-turned-starter Michael King.
There's a question of who the Tigers would bump from the rotation (or if they would go to a six-man) if they add to it, but they can still sign someone now and figure that out later.