"That's an incredible performance for him when all everybody ever wanted today was a homer from him,'' manager AJ Hinch said. "That's what everybody expected, and he did it.''
Alvarez, who was promoted from Triple-A Round Rock, served as Houston's designated hitter and batted fifth in Sunday's game against Baltimore right-hander Dylan Bundy. His first major league home run was a two-run shot to center field in the bottom of the fourth that traveled 413 feet.
He finished 1-for-3 with the home run and a walk in the Astros' 4-0 victory.
"That first at-bat, obviously, I felt a little bit nervous,'' Alvarez said through an interpreter. "But then I started thinking about all the players here and how much advice they've given me and how they made me feel comfortable, [and] I felt great.''
Hinch said before the game that Alvarez will see some time in left field, but he wants him to get a feel for Minute Maid Park first.
Alvarez, who turns 22 later this month, has dominated the Pacific Coast League this season, batting .343 with 23 homers -- the most in the minors -- and a league-leading 71 RBIs in 56 games. He has split time at first base and left field throughout his minor league career.
Alvarez, recently ranked baseball's No. 44 overall prospect by MLB.com, is the latest prospect to be called up to the majors by the injury-plagued Astros, who have All-Stars Jose Altuve, George Springer and Carlos Correa on the injured list.
The Associated Press contributed to this report.