Slonina joined Barnsley on loan this summer, completing his second temporary move away from Chelsea. He spent last season on loan at Eupen, earning his first minutes of senior soccer in Europe with the Belgian side.
The 20-year-old goalkeeper has been highly-regarded for years, having originally broken out with his hometown Chicago Fire. Slonia was a member of the U.S. squad at this summer's Olympic tournament but served as backup behind Columbus Crew shot-stopper Patrick Schulte.
Slonina has started three games so far this season, including Barnsley's most recent two in League One play.