I just asked ChatGpt
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Yes, there have been instances in Major League Baseball where a player hit three home runs in a game and was not in the starting lineup for the following game.
One notable example occurred on August 1, 2021, when Michael Toglia of the Colorado Rockies achieved the rare feat of hitting three home runs in a single game. Despite this impressive performance, Toglia was not in the starting lineup for the Rockies' next game. This decision was likely influenced by the team's strategic considerations, such as matchups, player rest, and overall team dynamics. mlbdailydingers.com
Another example is Seby Zavala of the Chicago White Sox, who became the first player in MLB history to hit his first three career home runs in the same game on July 6, 2021. Despite this remarkable achievement, Zavala was not in the starting lineup for the White Sox's next game. This could have been due to various factors, including the team's roster decisions and the need to manage player fatigue. espn.com+1mlbdailydingers.com+1
These instances highlight that even after a player delivers an outstanding individual performance, team management may opt to make lineup changes based on broader strategic objectives.”