"Unless somebody is injured our roster is set," Leyland said after the Tigers defeated Houston 10-0 at Minute Maid Park. "We're going to take Clete Thomas [and] the pitching staff is set with 12 pitchers."
Leyland informed Thomas and the two relievers who had been on the bubble, Yorman Bazardo and Aquilino Lopez, following Friday night's game.
"I told him to prepare yourself," Leyland said of Thomas. "I basically told him just to enjoy the ride, however long it is."
Thomas, 24, is batting .308 with a home run and six RBIs this spring. His presence became necessary when center fielder Curtis Granderson fractured a finger last Sunday.
Thomas had already left the visitor's clubhouse and wasn't available for comment.
"From what I've seen he's the best tools prospect that we have," Leyland said. "This guy's got a lot of ability."
Thomas will be the fourth outfielder on the Opening Day roster, joining Jacque Jones, Magglio Ordonez and Marcus Thames.
Bazardo and Lopez will be part of a seven-man bullpen that will also include Denny Bautista, Jason Grilli, Zach Miner, Bobby Seay and closer Todd Jones.
The rotation is set with Justin Verlander, Kenny Rogers, Jeremy Bonderman, Nate Robertson and Dontrelle Willis.
Ivan Rodriguez is the starting catcher, with Brandon Inge backing him up, who is also available for infield and outfield duty.
Rounding out the rest of the expected 25-man roster are infielders Carlos Guillen, Edgar Renteria, Placido Polanco, Miguel Cabrera, Ramon Santiago and Ryan Raburn and designated hitter Gary Sheffield.
Jim Molony is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.