At age 16, Javier starting pitching, having a maximum velocity of 86 miles per hour (138 km/h) when he initially auditioned for the Houston Astros.

Although his velocity was deemed to be low, the fastball generated notable whiff from batters and he threw with excellent command and control.

[12] Javier made his postseason debut against the Minnesota Twins in the American League Wild Card Series, also receiving his first playoff win in this game.

[13] Javier finished third in voting for the AL Rookie of the Year award behind Luis Robert and the winner, Kyle Lewis.

[16] Javier finished the 2021 season 4–1 with a 3.55 ERA, setting numerous then-career highs, including 36 games pitched, nine starts, 101+1⁄3 innings, 130 strikeouts, 53 walks, and two saves.

[18] Javier tied his career high with nine strikeouts on May 25, keeping the Cleveland Guardians hitless until Ernie Clement singled with two outs in the fifth inning.

[19] On June 25, 2022, Javier threw the first seven innings to lead his team to a combined no-hitter against the New York Yankees, starting opposite former Astros ace Gerrit Cole.

[24] On August 20, Javier hurled six innings of two-hit and one-run ball with eight strikeouts versus the Atlanta Braves for a game score of 71.

[25] Javier retired the first 13 Baltimore Orioles faced on September 25, allowing one total hit and baserunner over six innings while striking eight for a game score of 78.

[28] Javier concluded the 2022 regular season with a 2.54 ERA and 11–9 record over 30 total appearances, 25 games started, 148+2⁄3 IP, 89 hits allowed, 52 BB, 194 strikeouts and 3.7 Wins Above Replacement (WAR), all of which established new career-bests to that point.

[30] In Game 3 of the 2022 American League Championship Series (ALCS) at Yankee Stadium, Javier made and won his postseason starting debut as the Astros prevailed, 5–0.

[31] In Game 4 of the 2022 World Series at Citizens Bank Park, Javier delivered the first six innings of a combined no-hitter and 5–0 win over the Philadelphia Phillies.

[40] His father saw him pitch in Major League Baseball for the first time during a combined no-hitter started by Javier in Game Four of the 2022 World Series.