The player to be named is expected to be a good prospect. Since the Indians are not contenders this year, they used DeRosa to get younger.
DeRosa was having a great season with the Indians, hitting .270 with 13 home runs and 50 RBI. He can play anywhere in the field so the Cardinals will have no problem getting him in the lineup.
With Khalil Greene doing a pretty good job at third base my guess is that DeRosa will see the majority of his time at second base. It's also possible for him to play third and Greene go back to shortstop or back to the bench.
Since Brendan Ryan is dealing with tendinitis in his wrist, it's possible he finds his way to the bench for at least DeRosa's first week or two with the team. Ryan might even find his way to the DL now that they have another infielder.
No matter where DeRosa plays, he will be an upgrade. This is a huge boost to their playoff chances.