To do this, I am using ZiPS. ZiPS is a projection system created by Dan Szymborski, and it is constantly updated at FanGraphs throughout the season. As a result, we can estimate how a player will perform over the remainder of the season, based on his production to date both this year and in previous seasons. The next step is converting that production into runs above or below average, and comparing the various players already with the Pirates to those potentially available in a trade.
I decided to look at a few of the names that have been discussed over the past few weeks or so. This is meant to be a general look at the situation, so I included Carlos Beltran even though he has already been traded to the Giants. I estimated that each player would accumulate 225 plate appearances in an everyday role down the stretch (3.75 plate appearances per team game).
As you can see, all four external options would be upgrades over the players already under Pirates control. Generally, ten runs is equivalent to about one win, so the maximum upgrade in this case is around one win. A single win is pretty significant at this point in the season, particularly for a team in a close divisional race. The Pirates are currently on pace for 84 wins this year. Assuming they maintain that pace (this is far from certain, but in a hypothetical sense), bumping that up to 85 wins could certainly turn out to be significant. A one-win improvement is not very important for a team that wins 57 games; for one battling for a playoff spot, it is huge. In other words, the Pirates absolutely should be looking for ways to upgrade the offense for the stretch run.
However, is it worth blowing up the farm system for a one-win improvement? The system has improved the past few seasons, but the organization is not yet in the position where it can start shipping out loads of promising minor leaguers. There are still plenty of holes on the major league team that will need to be filled by prospects over the next few years. A one-win improvement could potentially be the difference that allows the team to sneak into the playoffs, but it does not guarantee anything. It may just take the Pirates from 84 to 85 wins, still leaving them a few games back in the NL Central race.
The Pirates should certainly be trying to upgrade their feeble offense over the next few days. But if you find yourself falling into the “trade whatever prospects it takes” mindset, keep in mind that any player coming back in return is not going to make this an above average offensive team by himself.