Rick sez,

I forget how young Walker is.  A mere 21. Man, that is young! Are there any other 21 year olds in a major league starting rotation? - 

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This is a point that Dr. D neglects constantly:  Taijuan's learning curve, his rate of progress.  Let's not forget that he's a native hoops player, not a ballplayer ... he got better so FAST that it almost seems unique.   Trevor Hoffman comes to mind, after the position switch.

Just his mechanics alone ... in about 1.5 seasons, his sports motion went from "ugh" to "major league worthy."  That's about 4 standard deviations' worth of progress that James Paxton still has not yet acquired.

Like Rick says, Taijuan pitched quite well in the majors last year, age 20 ... right there, in that alone, you've got something VERY interesting.  Even if his repertoire weren't so electric.

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Here is one article about age 20 starters.   Since the 1960's, there have been 18 rookie starters who, at age 20, threw 120+ innings.  That is less than one, every other year.  

Yes, "120 innings," that's different from Taijuan, but let's THINK rather than insisting on hackneyed methodology:  "throwing 120 innings" is not the only way to define "20-year old SP who is legit in the major leagues."  That is who the author was trying to identify, 20-year-old starters who were able to compete with big leaguers.  He chose 120 innings as a reasonable way to say "able to compete," which is ... a decent way to quantify it.  :: shrug ::

Nobody needs an arbitrary metric, in order to be able to say "Taijuan Walker, 2013, was a reasonable MLB rotation member."  He was.  Obviously.  Without any need for special Fangraphs expertise in the saying so.  

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Of those 18 rookie starters, over the last 45 years, the list is packed with guys like Valenzuela, Saberhagen, Greinke, Sabathia, Kershaw, etc.  (Of course, Felix also threw 120+ innings at age 20, but the article is about rookies.)

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Like we sez, it is really just a question of Taijuan's health.  It would be a major upset if a healthy Taijuan were NOT a top of the rotation starter.  But then, the scouts coulda told you that a long time ago...

Remember too, "Major League Equivalencies" are based on the idea that --- > although Jesus Montero has a 30% chance to not hit his MLE's in the following year, so does any given major leaguer, such as Chone Figgins.  Taijuan could fail in 2014, but then so could Ubaldo Jimenez.

You are free to speak of Taijuan Walker as "our young Bret Saberhagen."  Taijuan isn't a "prospect" in the sense that D.J. Peterson is a "prospect."  Neither was Masahiro Tanaka.  There are a few non-MLB players who are as good a bet as the current MLB'ers are.

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Like Matty sez, we are replacing some supah-ugly #5 SP's with our young Bret Saberhagen here.  Not happy news for the Yankees.

Enjoy,

Dr D