The Cardinals acquired Armstrong from the Rays on Tuesday in exchange for outfielder Dylan Carlson, Ken Rosenthal of The Athletic reports.
Armstrong was dominant in 2023 with a 1.38, 0.90 WHIP and 54:11 K:BB over 52 innings, but he's labored this season with 5.40 ERA in 38 appearances. The right-hander should primarily work in middle relief for the Cardinals.