Herrera had a .920 OPS in the Frontier League in 2012. League average is .716, as the league has a slugging percentage of just .379, while the batting average is similar to MLB averages and the OBP is higher. Of course, finding the context for these statistics from Herrera will be difficult, if not impossible. Park Factors, as far as I can see, are impossible to find, platoon/ballpark splits are unavailable (not to mention he played for two different teams), and we (or I don't) simply don't know how independent league statistics translate to the minors or big leagues. There does seem to be quite a deviation though, as either there is a great competition gulf between teams or some pretty extreme park factors. In the Majors in 2012, the worst offensive team was the Astros who scored 3.60 runs per game. The Rangers were the best team, and they scored 4.99 runs per game (Colorado gave up 5.49 runs per game, while Tampa Bay gave up 3.56 runs per game). In the Frontier League, the best offensive team scored 5.95 runs per game, while the worst offensive team scored 3.40 runs per game, a bigger difference obviously. It may be safe to say that the Rockford RiverHawks (Herrera played for the Southern Illinois Miners and RiverHawks) played in an offensive friendly environment, as they scored 5.01 runs per game and gave up 6.19 runs per game. Of course, the alternative explanation would be that their pitching was horrible but they had good hitters. The pitcher that lead the team in starts (Cody Hallahan) was 24 years old and has no stats in either college (at least not a Major college that would show up on Baseball Cube) or the minors (or even any independent league stats before 2012). They simply may have just had bad pitchers (they also had Jimmy Parque on offense, who has been signed by the Cardinals I believe) or even a bad defense.
To be fair, the Frontier League website does have some advanced statistics available, and Herrera was 4th in the league in runs created, the best out of outfielders. His K/BB was unimpressive, but this was this the case in the minors as well. His ISO of .204 wasn't overly impressive either, ranked 22nd from my count. His 93 games don't sound like a lot, but teams only play 96 games a year in the Frontier League, showing that he was finally able to stay healthy. That number is more meaningful to me than any of his actual statistics in 2012.
I wanted to see if there was any change in Herrera at the plate, and videos of him in both the minors and in the Frontier League were easy to find, so I shared them below:
Swing from '07:
Swing from 2012:
The two swings seem basically the same to me, he keeps his hands up high and then dips his shoulder before driving the ball with an uppercut swing. While he shows good control with the bat, the overall mechanics are not exactly pretty.
Defensively, ROTZ data liked him in the outfield when he was in the minors, playing mostly centerfield, though FRAA wasn't much of a fan. I wouldn't trust range factor data from the independent leagues (though perhaps minor league data probably isn't much better). However, it is extremely likely that the injuries have sapped Herrera's speed and defensive skills at least somewhat. In 2008 (in AA Midland), he had a 6.2 speed score, and according to the crude speed score metric I used in the KBO WAR, he had a 5.33 speed score in 2012, not running a lot, but not getting caught a lot. This would suggest a regression, but only a slight one and he would still be an above average baserunner.
Already 27, with 5 career games in AAA (back in '05!), the corner outfielder's likelihood of impact is obviously small to non-existent just based on odds. He has never had a season of more than 14 homers at any level, which is not something you want to say for a corner outfielder, even a good defensive one. There should be place for him in AAA, but whether he provides any kind of value depends on how much his defensive/baserunning skill set has eroded.