Recently I wrote about Ivan Herrera as a potential trade candidate. It may seem weird for a team at the bottom of the NL Central to trade a top 5 prospect but with the Cardinals likely pursuing a retool to remain competitive next year (and potentially this year if they can turn things around in a weak division), it could make sense.
The reasoning is simple. His path to playing time is blocked, at least in the near future, and he could bring back a good return as (likely) the only top 5 prospect the Cardinals would be willing to trade.