Andrew Kittredge
He's not Ryan Helsley, but Andrew Kittredge has been the definition of a stopper for the Cardinals this year. Whenever the Cardinals have been in a pinch or are looking to hold a lead at any point in the game, Kittredge has been available to run out to the mound and get them three outs.
In his 15 appearances for St. Louis thus far, Kittredge has a 1.20 ERA and 0.93 WHIP while striking out nine batters per nine innings. Kittredge has proven he's back to being the All-Star reliever he once was, and any contending team would love to add him to the back of their bullpen.
Kittredge could slot in as a team's closer, set-up man, or the guy they bring in during any inning to get them out of a jam like the Cardinals do. That kind of reliever is valuable on the trade market, and would surely bring the Cardinals back a nice return.
For context, Aroldis Chapman was traded by the Royals to the Rangers for Cole Ragans last season, and now Ragans is emerging as an ace for Kansas City. The Cardinals got Sem Robberse in the package they received for Jordan Hicks, and he's been performing at a high level for the Memphis Redbirds this season. The Cardinals gave up Richie Palacios to go get Kittredge this offseason, and I expect them to be able to get that kind of return or better at this year's deadline if they decide to move him.
Kittredge is a free agent following the season, so he would be just a rental for any contender in the market for him.