Foster was optioned to Triple-A Charlotte on Friday, James Fegan of The Athletic reports.
The right-hander impressed as a rookie last year with a 2.20 ERA, 0.87 WHIP and 31:9 K:BB across 28.2 innings, but he's struggled to find that form in 2021. Foster has a 6.15 ERA this season and will head to Triple-A in an attempt to straighten things out.