RHP Zach Willeman returns for a second season in Sioux City after spending parts of five seasons with the Los Angeles Dodgers, Kansas City Royals, and Miami Marlins organizations. In 2024, Willeman excelled in a setup role, going 5-4 with a 3.42 ERA and a 1.160 WHIP. He struck out 67 batters in 50 innings, allowing 19 earned runs on 28 hits with 30 walks. Willeman made 10 consecutive scoreless relief appearances from July 31 to August 17 and finished second on the team with 14 holds.

Willeman joined the Explorers in 2024 after a year with the Miami Marlins organization. In 2023, he pitched in 14 games for AA Pensacola. In 2022, he split time between AA and AAA for the Royals, posting a 5.16 ERA at AA Northwest Arkansas and a 3.29 ERA at AAA Omaha. Drafted by the Dodgers in the 19th round of the 2017 MLB Draft, Willeman began his professional career in rookie ball and Single-A.

Willeman played college baseball at Kent State, where he was a standout reliever and starter. He attended Napoleon High School in Ohio and posted a 0.63 ERA with 121 strikeouts as a senior.