The first thing that would need to be done to understand if BC could be a good starter or not is to project what his future line would be.
First thing to understand his his plus feature in his offense is his power. He has a career MILB ISO of .213 (ISO = Isolated Power, SLG minus AVG). His MLB career ISO is only .106 but we're also talking about a grand total of 133 PAs. He does have poor contact rates which will hurt his power numbers but he's also in the prime of his career...but he also plays in a pitchers park. Projections have him at around .170-.175 ISO, so let's go with .175.
Next part is OBP, or walk%. His MILB walk% is 6.8% and his MLB walk% is 6.8%. Yeah.
The problem next is average. He has a bad MILB average (.259), a horrible MLB average (.220), bad BABIPs, bad K%, bad contact%, the list goes on. Plain and simple, unless some drastic change happens, he will not hit for average. Projections have him as being a .230-.240 hitter. Because we're marlins fans and we want to be optimistic lets say .240.
That would put up at roughly a .240/.295/.415 line, or a .710 OPS.
I've said it before but I think he could more or less be our Franklin Gutierrez. Gutierrez has a career line of .260/.314/.408/.722. Better contact but less power. However, Gutierrez has been a good player because of his defense, throughout his career he's been worth 5.9 wins. His career is about 2 seasons worth of stats, so he's worth roughly 3 wins over a season, which is very good.
The problem though is that Brett Carroll is not Gutierrez defensively. Carroll has a plus arm and plus range, but Gutierrez has possibility the best OF range in all of baseball. A more apt comparision for Carroll is probably Alex Rios. There might be better ones out there but he's the best I can think of at the time. Plus, Rios is a pure RFer. Rios
Rios in his career has a 14.3 UZR/150 (or, in other words, he rates to have a 14.3 UZR in a 150 game span). He save 6.9 runs with his arm, 6.9 runs with his range, and 0.5 runs by not creating errors.
The problem with Carroll's career fielding numbers is that it's such a small sample size. He has less than 300 innings played in the OF. I mean, just one season is a small sample size when it comes to fielding stats. Regardless, he has a career 34.5 UZR/150. 7.4 of that comes from his arm, 26.4 comes from his range, and 0.7 comes from preventing errors. Now that range will come down a lot, but his arm and error should rate out about the same.
So now we have Brett Carroll's projected offensive line (.725 OPS) and his projected defensive line (+14 runs).
The problem ofcoarse being the insert Fielding Stats are far from perfect statement here. They only show probably 30-50% of the picture. It's by no means definitive. However it's a lot better than blindly grabbing at the dark.
His offense would be worth around -2 to -5 runs compared to the average player depending on just where exactly he falls. It's probably at about -3. With his defense, that means he's now at +11 runs. Take on the worth above a replacement level player (18.3) and positional adjustment (-6.5) and carroll would be worth 22.8 runs, or about 2.3 wins, over a replacement level player.
This is basically around the same worth of a .800 OPS neutral defenseive RFer, meaning his defense is worth nearly .100 OPS points.
By no means a star but still a good starter. And it does give us the ability of, moving CC to the IF, trading Hermida, ect without worrying about who'd start. It doesn't seem to me like BC will get that privledge but he would probably do well enough in that roll.