I don't know for sure but, here is my guess...
Jason Kubel, Delmon Young , Craig Hansen and BJ Upton
B.J. Upton- Still has some defensive issues to sort out. With Lugo still in Tampa for the time being I don't forsee them rushing him to the majors just yet. Keep in mind he is still only 21 years old.
Delmon Young-Still only 20 and TB is pretty good in the OF for the time being. That being said expect him to be called up at some point this season either due to injury or a hot streak.
Jason Kubel- I think the same situation as Delmon Young. However, I think he stands a better chance then Young to make the team out of ST.