New York Yankees
The New York Yankees are making a serious playoff push this year. They've been battling the Baltimore Orioles throughout the first month of the season for the top spot in the American League East. Their playoff odds currently stand at 92.3% according to Fangraphs, so they're a virtual lock at this point.
A strong bullpen will help the Pinstripes shorten games even more. Their bullpen currently has the 2nd-best ERA, but they are only striking out 8 batters per nine innings. Also, they're currently 11th in fWAR as a group. Clay Holmes is near the top of the league with 12 saves, so it will be tough to replace him as the team's closer. Helsley would help strengthen an already strong group for the Yankees.
Holmes and Helsley have complimentary splits; Holmes is better against right-handed batters while Helsley has had more success against lefties. The two could work in tandem and switch out late in games depending on which batters are due up for the opponent.
In exchange, the Cardinals could call for one of the Yankees' top-100 prospects. Chase Hampton is the Yankees' lone pitcher left in the top 100 after their trade for Juan Soto. He's an enticing player, but he has been hurt all year with shoulder discomfort. When healthy, Hampton projects to be a #3 starting pitcher, something the Cardinals could use in their rotation. He features a plus fastball that can peak around 97 MPH, a tight slider with great break, and a strong cutter. Hampton's arsenal is deep, as he throws 5 pitches in total.
A Helsley-for-Hampton trade would be interesting. He's currently 22, and he last appeared at Double-A for the Yankees. He pitched 59.2 innings there with a 4.37 ERA, a .240 batting average against, and a 68-21 strikeout-to-walk ratio. Hampton would be another pitcher with decent upside for the Cardinals to go along with Cooper Hjerpe, Quinn Matthews, and Sem Robberse.