Let’s start with this idea: If a player is ready to step up, it will show in spring training.
You could validly point out limitations of small sample sizes, some spring training players are AAA, and the importance of in-person observation. On the other hand, performing well in spring training reflects excellent offseason preparation and performance consistency in all circumstances.
Let’s look at position players who could step up if a starter is injured. Let’s assume each player has a primary position that they could best step into (other positions would hurt the team defensively).