Slonina suffered relegation with Eupen last campaign, but made 34 appearances for the Belgian side in his first full-season in Europe.
The 20-year-old joined the Blues in August 2022 from MLS side Chicago Fire, but has yet to appear for Chelsea's senior team. He was loaned back to the Fire for the remainder of the MLS season in 2022, finishing the season with 22 matches played. He would join Chelsea's academy side through the remainder of the 2022-23 campaign before being sent on loan to Eupen. Slonina has one senior appearance with the U.S. Men's National Team.
Posting on social media, Slonina exclaimed his excitement to secure the move, saying he's "happy to be here," while posing at Oakwell Stadium in a series of photos.