I know he's 21, but I'll look at this objectively, rather than overrate every Red Sox prospect.
A prospect who can't throw secondary pitches, can't cut it in the big leagues. He probably could be an effective reliever, but a number three starter? I'd doubt he could go even twice through a lineup without being pounded.
Wait a minute, an idea just hit me. I'm only 16 years old, if I throw for five years, I'll be good enough to be a 3rd starter at the highest level. I'm exaggerating of course, but the point is that some people can't throw baseball pitches, no matter how hard they work.
The other point, I'm trying to make is that lefties who throw with low velocity and don't have pinpoint control, will be feasted on when they hit the bigs. Looking at Phillips control, it's below average. Thirty-one walks in 67 innings equates out to about 93 walks in 201 innings. For a player like Randy Johnson, that would be OK, but a lefty with substandard stuff needs to avoid as many baserunners as possible. He can improve on that, but how much improvement do you really expect?
You guys keep bringing up the point that he's fastball is automatically going to improve. That's not assured. Just take a look at one of our own pitching prospects, Abe Alvarez. He's been throwing 87 mph (if he's lucky) since 2003.
Realistically, calling Phillips a number three starter is overstating it. Look at Alvarez, he hasn't shown he can get throw a major league lineup more than once, and the only way he's going to wind up in the bigs, is as a relief pitcher. I'd expect the same from Phillips, no way anyone else should expect any different.