The
Cincinnati Reds signed Spiers as a
free agent after he went unselected in the shortened
2020 Major League Baseball draft. However, he did not play in a game due to the cancellation of the minor league season because of the
COVID-19 pandemic. Spiers made his professional debut in 2021, spending the year split between the Single–A
Daytona Tortugas and High–A
Dayton Dragons. In 25 total games (20 starts), he registered an 8–4 record and 3.55 ERA with 130 strikeouts across innings of work. In 2022, Spiers split the season between Double–A and the Triple–A
Louisville Bats. In 27 games (23 starts) between the two affiliates, he posted a 4–6 record and 5.33 ERA with 105 strikeouts across innings pitched. After alternating as a
starting pitcher and relief pitcher early in his career, Spiers became a full-time relief pitcher with the
Chattanooga Lookouts of the
Double-A Southern League in 2023. On September 1, 2023, the Reds promoted Spiers to the major leagues as a
COVID-19 substitute. He made his MLB debut on September 3. In two starts for the Reds, he allowed seven runs on ten hits and five walks with eight strikeouts in seven innings pitched. On September 10, the Reds returned Spiers to the minor leagues. On September 14, the Reds selected Spiers to the
40-man roster and promoted him to the major leagues. In four total appearances for Cincinnati, he logged a 6.92 ERA with 12 strikeouts across 13 innings pitched. Spiers was optioned to Triple–A Louisville to begin the 2024 season. He made 22 appearances (10 starts) for Cincinnati during the regular season, compiling a 5-7 record and 5.46 ERA with 80 strikeouts across innings pitched. Spiers made three appearances for Cincinnati to begin the 2025 campaign, struggling to an 0–2 record and 6.08 ERA with 11 strikeouts across innings pitched. On April 20, 2025, Spiers was placed on the injured list with a right
shoulder impingement syndrome. He was transferred to the 60-day injured list on May 23. On July 26, it was announced that Spiers had suffered a
UCL injury during his rehabilitation, which necessitated season-ending surgery. He was
designated for assignment by the Reds on November 18. On November 21, Spiers was non-tendered by Cincinnati and became a free agent. On November 24, 2025, Spiers re-signed with the Reds organization on a minor league contract. ==Personal life==