There have been no rumors (to my knowledge) when it comes to starters in which the Cardinals may be interested. And honestly, there's no one out there worthy of giving up the wealth of the Cardinals' farm system.
But here are some names who seem to be on the trading block who could improve the rotation:
Ricky Nolasco, RHP (Miami Marlins)
Right now, Nolasco is the most sought-after starter on this year's hot stove. He currently holds a 3.68 ERA, and his strikeout-to-walk ratio is historically very good.
The Cards might have to give up a decent prospect or two, but it would be nice to have a seasoned middle-of-the-rotation kind of guy for September. Yet they'd have to move quickly because the Dodgers and other clubs might also be interested in acquiring Nolasco.
Bud Norris, RHP (Houston Astros)
Norris has been a bright spot on a last-place team. In 2013, he has a 3.60 ERA despite a 5-7 record. Imagine what he could do under the tutelage of pitching coach Derek Lilliquist. The Astros have been shopping him since 2012, so the competition for his arm will be fierce. But if anyone has the minor league depth to pull it off, it's the Cardinals.
Jake Peavy, RHP (Chicago White Sox)
When healthy, Peavy can be the ace on many staffs. Maybe not the Cardinals', but he can fill the void should Miller's innings load be curtailed.
He will be making a comeback post-All-Star Game from a rib injury. If the Cardinals can swing something, he might be the best option of the group. The only risk would be his injury-prone past. The Cardinals wouldn't want to pay for another damaged starter after losing Chris Carpenter and Jaime Garcia.