The difference between selling and buying baseball players on July 31?

Jeff Luhnow is about to find out.

For the first time in four seasons, the Astros’ general manager will be shopping instead of getting rid of used pieces as the sport’s non-waiver trade deadline approaches.

A 52-43 club that sits in second place in the American League West, two games behind the Los Angeles Angels, needs sharper starting pitching and better bats during the last two months of baseball’s six-month marathon. If the Astros, who prize a farm system that’s been overhauled into one of the game’s best, can only make one major team-altering decision at the deadline, the answer belongs on the mound.

Luhnow’s club ranks fourth out of 30 teams in batting average against (.239) and 10th in ERA (3.56), while being first in offensive home runs (129), third in runs scored (422) and ninth in on-base plus slugging percentage (.731). But as tempting as it would be to add a proven run producer in the middle of a streaky lineup that tops the majors in strikeouts (863) and is the worst in ground ball to fly ball percentage (.76), the Astros have proved since April they can somehow live a good life with Chris Carter (.190 batting average), Luis Valbuena (.205) and Jason Castro (.206) combining for 806 at-bats.

What the Astros won’t be able to get by without in September is another top-of-the-rotation starter to pair with surprising ace Dallas Keuchel. Outside of Collin McHugh (12) and Keuchel (16), no one on the team has more than seven quality starts or 84.2 innings pitched this season.

Outside of Keuchel, none of the Astros’ arms have been consistently accurate since spring training was complete. And as promising as rookies Vincent Velasquez and Lance McCullers have been since being called up from the minors – the duo have posted a combined 5-4 mark with 109 strikeouts to 38 walks in 102.1 innings – the young duo will be monitored and protected at the same time as the Astros are playing their most important games since the franchise’s lone World Series appearance in 2005.

Whether it’s Cole Hamels, David Price, Johnny Cueto, Jeff Samardzija or any of the other names that will be floated during the next 10 days, the Astros will have multiple mound options as July 31 nears. Adding a single new arm would change the team’s season, turning baseball’s biggest 2015 surprise into a true postseason contender.

“The biggest impact would probably be against the Angels, who we play nine more times. Being able to have that one-two punch if we can line it up,” Luhnow said Tuesday. “And then clearly if we get to a best-of-five playoff series, you really need three starters to get through that.”

The Astros could use a bat in August and September.

They can’t live without an additional arm.

The last five World Series winners (San Francisco three times, Boston, St. Louis) have been defined by loaded teams that became hot late and played their best baseball in October. The Giants, Red Sox and Cardinals also had premier starting pitching to carry them through moments the Astros haven’t experienced in a decade.

Keuchel's carried the Astros' arms all season. He can't be asked to go it alone when the leaves begin to fall.

“The Dodgers are going to have (Clayton) Kershaw and (Zack) Grienke and the Nationals are going to have a great – as many as they want, really,” Luhnow said. “I do believe Keuchel is as good as anybody we will face. But when you have teams that have a (Steven) Strasburg or Grienke as their second starter, we like our guys. But we’d like to have somebody that matches up better in the second spot.”