Gary Gibson, who made 26 starts with the Rams in three seasons, is Miller's top competition. Gibson has missed some practice time with an injury. Amobi Okoye, a free agent from the Bears, will also be in the rotation and figures to play on passing downs. He sat out Thursday to rest his knee after offseason knee surgery.
While Miller is the favorite, coach Greg Schiano will use the preseason games to help him make a decision of who plays next to star defensive tackle Gerald McCoy.
"As we work through it, Gary, Roy, Amobi and then whoever else we throw into that mix, it's just going to be competition," Schiano said. "Whoever performs the best will get the bigger chunk."
Several Bucs sit out practice: First-round pick Mark Barron missed Thursday's practice with a toe injury. Defensive end Michael Bennett left practice early on Wednesday and did not practice on Thursday, according to the Tampa Bay Times.
The Bucs also sat safety Ronde Barber for most of practice on Thursday to rest his 37-year-old legs.
Follow Bucs reporter Patrick Southern on Twitter @CBSSportsNFLTB.