Francisco Liriano is unlimited potential, contained within a 6-foot-2, 215-pound frame and a left arm capable of whipping and spinning a baseball in ways mere mortals can only imagine. Only such potential could have earned a two-year contract following his last two seasons -- a 5.23 ERA over 52 starts, plagued by home runs and wildness -- and only such potential could still guarantee $1 million even after the club found a broken non-throwing arm in his physical.

The Pirates wanted him bad enough to stick with him despite an injury guaranteed to keep him off the opening day roster, and despite prior incompetence that would relegate a typical pitcher to clawing through camp on a non-roster invitation. Liriano, 29, has rewarded them handsomely. His latest masterpiece -- seven shutout innings and 13 strikeouts against the Padres -- pushed his ERA down to 2.53, between Patrick Corbin and Adam Wainwright among National League leaders. His 14 wins are tied for the league lead despite the missed time. His 2.66 FIP trails just Wainwright and Clayton Kershaw.

Score one for potential.

But even with Liriano's thrilling comeback year, his career remains one of the great what-ifs of baseball's current era. What if Liriano hadn't tried to pitch through pain in August 2006, as the Twins were rolling towards a playoff spot? What if he had told Twins bullpen coach Rick Anderson or manager Ron Gardenhire about the pain he had felt while pitching, or during a bullpen session before his second-to-last start of the year.

From The New York Times:

"I should have said something; that was my fault," Liriano said Thursday while standing at his locker with an ice wrap around his elbow. "I didn't feel right to pitch, and I went out to pitch. That's not O.K."

Liriano started one more game, a month later. He lasted two innings. In November, he underwent Tommy John surgery. He didn't return to the majors until midway through the 2008 season.

But what a run he had. The Twins finally moved Liriano out of the bullpen May 19th for a start against the Brewers, and he ran with the opportunity. He held Milwaukee to one run on two hits over five innings, struck out five, and drew 10 swinging strikes on just 68 pitches. He didn't allow another run until the fourth inning of his fourth start. On June 16th, he struck out 11 in a win over Pittsburgh. Then he struck out 12 to defeat the Brewers again on July 2nd, and 10 to defeat Cleveland on July 23rd, and 12 in a loss to Detroit -- despite eight innings pitched and just two runs allowed -- on July 28, his final full start of the season.

Liriano completed five scoreless starts, six without an earned run, and 12 of 14 with fewer than three runs allowed. Over these 14 starts, Liriano pitched 92.2 innings with a 1.65 ERA and 105 strikeouts. Hitters posted a .162/.231/.251 line against him. The worst qualified hitter in 2006, the Rockies' Clint Barmes (now Liriano's teammate in Pittsburgh), hit .220/.264/.335.

What Liriano did would have been amazing in any era, but 2006 was one of the best offensive seasons in history. Of post-integration seasons, only 2000, 1999 and 1996 featured a higher league ERA than 2006's 4.53, and only 2000 and 1999 featured a higher league OPS than 2006's .768. This year, just 15 qualified starting pitchers have an ERA above 4.53, and just 71 qualified hitters have an OPS above .768.

Liriano domination of the American League in 2006 was akin to what Pedro Martinez did in 1999 or 2000, when he posted a 1.90 ERA over 430.1 innings against the best offenses of all time. It was only 14 starts, but Liriano had done nothing but improve until the injury shut him down.

"Where do you want to start?" Martinez's former teammate Derek Lowe said when asked to compare the two. "Liriano is very similar as far as velocity, throwing any pitch at any time. When you have that kind of stuff, it's easy to be as poised as he is."

"If I was the Minnesota Twins," former Twins great Jack Morris told the Minneapolis Star Tribune in July, "I don't know if there's anybody in baseball I would trade him for. That's how much I think he means to this organization."

The question never could have been about Liriano's talent.

He regularly tickled the upper 90s on the radar gun. His sliders could start at a left-handed batter's hip and end on the outside corner. His changeup, just like teammate Johan Santana's, seemed to have time-bending capabilities. And even with the absurd velocity and spin his arm imparted, Liriano always threw strikes. He walked just 28 in those 14 starts and walked more than three just once.

Morris, however, was prophetic when he said, "Durability is the one thing we don't know about him yet."

Durability has proved a constant enemy. Since the Tommy John surgery, Liriano has dealt with the following, according to Baseball Prospectus's injury database: left forearm swelling; left shoulder inflammation; left shoulder soreness; left shoulder strain; left arm fatigue; right thigh contusion; right upper arm fracture. Since that shortened start on Aug. 7, 2006, Liriano has missed 432 days to injury and has surely pitched through minor nicks on many others.

For me, Liriano still represents and always will represent unlimited potential. His 2006 was too captivating, too great and too full of possibilities to forget.

I was a junior varsity baseball player then, and my mornings before practice invariably found me glued to my TV, watching the previous night's highlights. Liriano's were arresting like none other, because of his ability, because of his sudden ascendance, because his pitches seemed to violate the laws of physics. I wondered for years if we would ever see it again. He teased us occasionally -- an encouraging return in 2008, brilliant peripheral numbers in 2010 -- but he was never as good, and never for a sustained period of time.

I've been sure for a couple of years now we wouldn't see it again. If anything, 2013 has proved we won't. Liriano's comeback has been stupendous, and his numbers would place him in the National League Cy Young conversation if he hadn't missed the first month of the season. He's a rotation rock for a Pirates team barreling towards the playoffs. And yet he still has not approached his 2006 level -- this year, with run scoring half a run down from seven years ago, Liriano's ERA is nearly a full run higher.

Liriano's 2013 has been great in its own way, of course, as a stunning comeback propelling a once-lost franchise to its first postseason in over two decades. Maybe that's the greatest testament to Liriano's potential: At the unlikeliest moment, here he is, captivating us one more time.