Needless to say this is a subjective ranking, but here is how I rank the six players the Pirates can least afford to lose and why as they push for the playoffs in the second half.
1.) A.J. Burnett: If Wandy Rodriguez weren't out (and in my non-medical opinion it's not unreasonable to think he may miss the rest of the season) I may not have A.J. first. But with the current situation, he heads the list. The Pirates have lost their #2 and #3 (James McDonald) starters and while their depth has paid off in a big way, losing the ace would be a huge blow.
2.) Pedro Alvarez: We've seen over the last month how his bat in the middle of the order is a game-changer. The Pirates don't have anyone close to having that kind of power in their line-up and no obvious replacement at third.
3.) Russell Martin: Neal Huntington's best free agent signing, Martin's defense has been a massive upgrade. His offense has also been a huge plus. If he were to go down, the drop-off to Michael McKenry and Tony Sanchez would be significant.
4.) Francisco Liriano: This is probably going to cause some argument and you could probably put Jeff Locke here just as easily. I chose the hard-throwing Liriano for much the same reason I have AJ up top. If Wandy were healthy he probably wouldn't be on the list, but here he is.
5.) Andrew McCutchen: Another spot likely to create tons of disagreement. Cutch's loss would be devastating, but Starling Marte's emergence would ameliorate it to some degree, their numbers very similar in the first half. This doesn't in anyway suggest Andrew McCutchen isn't the best player on the team--he most-certainly is.
6.) Starling Marte: See Cutch above. A great first-half in his first full season, Marte is a stud.
The most-obvious name left out is probably Jason Grilli. He's been as good as anyone in baseball. But the Pirates bullpen as a whole is so deep and has been so good I think they could survive the loss of any one reliever reasonably well.