We hope the Cardinals don't acquire a big bat
Acquiring a big bat at the trade deadline such as Brent Rooker or Luis Robert Jr. would go counter to what the Cardinals truly need at the deadline. While a platoon bat for center field may be necessary as insurance for Tommy Edman, who has still yet to play a game this year, the Cardinals don't really have room to add another bat to their lineup. They certainly could use one, but their two weakest positions other than center field are first base and third base, and they can't exactly move on from the future Hall of Famers they have at those spots.
Of course, fans have speculated that the Cardinals may be better off without Paul Goldschmidt at first base, opening either right field or first for a right-handed slugger, but that's entirely unrealistic. The high-end options available in center field are also way out of the Cardinals' price range. For now, it's probably just Luis Robert Jr. who will demand a prospect package the Cardinals simply don't have. Then, we get into crazy speculation of a Mike Trout trade, which just won't happen at this point.
While fans may want the additions of Brent Rooker, Pete Alonso, or Vladimir Guerrero Jr., the Cardinals simply don't have any space for them with the emergence of Alec Burleson and great position player depth throughout the roster. Unfortunately, they'll more than likely be banking on the resurgence of Goldschmidt or Arenado as their only meaningful offensive upgrades at the deadline. It's not going to be pretty, but it's all the Cardinals can do without completely depleting the farm.
The Cardinals could definitely use another bat, and will probably get one to platoon with Michael Siani, but asking for anything more than that is unfortunately unrealistic and unwise. The Cardinals can certainly seek lineup upgrades once Goldschmidt walks in the offseason, but for now, they can't afford to part with a franchise cornerstone in the middle of a contending season, and there's no other spot for an impact bat.