Just like Lowrie, Altuve is a great player on the Astros, a struggling team. And just like Lowrie, Altuve could be traded.
While it isn't likely, it could happen if the Giants give away a lot of prospects. The All-Star is hitting .303 with five homers and 27 RBI, and he would definitely be an upgrade over Ryan Theriot.
Melky Cabrera is on fire and Buster Posey is hitting very well, and if Altuve got on base at the top of the order, they could drive him in. Cabrera is hitting .353, and Posey is hitting .289. If you add Gregor Blanco or Justin Christian to the top of the lineup, the Giants would have a very dangerous offense.
And they wouldn't strand as many runners.
The Giants would have to give away a few prospects, and they would probably have to part ways with Gary Brown, the star outfielder who is lighting it up in Double-A. They may also have to give away a top pitching prospect, as the Astros are very attached to Altuve and probably won't trade him.
Even if the Giants traded a lot of prospects, they would still get a great player. Altuve is young and would be a Giant for a while, and if the Giants kept Cabrera, he, Cabrera and Posey would wreak havoc on opposing pitchers.
Again, it will take a lot to get Altuve. However, Theriot isn't a great second baseman, and if the Giants really wanted to upgrade their offense, they need to trade for a middle infielder. If they wanted to take the next step to contending for a championship, they need to work out a deal with the Astros.