The genesis of this article started with a tweet. This tweet in particular:

I don't know this for sure, but it sure feels like the Tulo/Donaldson left side is historically good. — Eric Garcia McKinley (@garcia_mckinley) August 3, 2015

Before the Colorado Rockies traded Troy Tulowitzki to the Toronto Blue Jays, young third baseman Nolan Arenado played to the right of shortstop Tulowitzki. If the Rockies didn’t trade Tulowitzki and they stayed on the left side of the infield together, they could have been historically good. Since the trade, vain bodybuilders probably have a monopoly on "historically good left side" chatter in Denver. That conversation as it relates to baseball should have migrated with Tulowitzki, as he joined third baseman Josh Donaldson in what, after some investigation, really has the chance to be a rare partnership on the left side of the infield.

Since 2013, Donaldson’s first full season, Tulowitzki and Donaldson have been remarkably similar. In over 1800 plate appearances, Donaldson has hit .282/.363/.504. His 141 OPS+, which adjusts for his time at the hitter unfriendly O.co Coliseum, indicates that he’s hit 41 percent better than league average since the beginning of 2013. He’s also demonstrated a good approach at the plate, which is evident in his 10.6 walk percentage and 18.1 strikeout rate. Both of those figures are better than league average since 2013. All the while, Donaldson has been good on defense. His Defensive Runs Saved is at 40. All of that combines for 20.4 fWAR—or, about 6.5 fWAR per 600 plate appearances.

In Tulowitzki, the Blue Jays added one of the best players in baseball to play next to another of the best players in baseball. Tulowitzki’s triple slash since 2013 is .314/.390/.536 in almost 1300 plate appearances. As opposed to Donaldson’s experience with a hitter unfriendly home park, Tulowitzki’s slash line is a bit inflated by his time at the hitter friendly Coors Field.

Tulo's 140 OPS+—40 percent better than league average—is pretty much equivalent to Donaldson’s 141. Additionally, Tulowitzki’s walk and strikeout rates are almost indistinguishable from Donaldson’s: he’s walked 10.6 percent of the time and struck out 17.4 percent of the time. At the tougher defensive position, Tulowitzki’s DRS is 14 since 2013. The 12.5 FanGraphs WAR he’s accrued translates to 5.8 fWAR per 600 plate appearances.

They are among the best in baseball, and now they play aside one another. But to be able to call the Blue Jays’ current left side historically good, we have to find some historical comparisons. To identify them, I used Baseball Reference’s Play Index. I searched for shortstops and third baseman on the same team since 1961 who played at least 100 games and at least 90 percent of those games at those positions. To find players whose value came at the plate and not just with the glove, I set an OPS+ limit of 115.

There were some pretty great left side combinations, though the search only yielded 29 such seasons since 1961. From 1982-1984, the Milwaukee Brewers had a Hall of Fame left side of the infield. In those seasons, Paul Molitor manned third base while Robin Yount played shortstop, but 1982 was the only of those three seasons when both posted an OPS+ over 115. Molitor’s 1983 OPS+ fell below 115, and his 1984 was injury ridden.

After that, positional changes split the left side apart, though they remained teammates until 1992. For two seasons, in 2006 and 2007, third baseman Miguel Cabrera and shortstop Hanley Ramirez handily exceeded the 115 OPS+ threshold, but they played for the Marlins, so that was never going to last. Jose Reyes and David Wright formed an excellent left side for the Mets from 2004-2011. Reyes’s bat, however, was usually closer to league average, so they only met the criteria twice, in 2006 and 2008. A less expected pair who did it in multiple seasons were Miguel Tejada and Melvin Mora for the Orioles in 2004 and 2005.

But there’s a much more formidable left side—one, in fact, that owns four of the 29 seasons that met the performance criteria above. Obviously, it’s Alex Rodriguez and Derek Jeter. In 2005, 2006, 2007, and 2009, both Rodriguez and Jeter posted OPS+’s above 115. It’s important for our search for historical comparisons to Donaldson and Tulowitzki that those were Jeter’s age 31-33 and age 35 seasons; they were Rodriguez’s age 29-31 and age 33 seasons.

Next season, Donaldson and Tulowitzki will enter their age 30 and 31 seasons, respectively. Given both players contracts, they are set to have three full seasons together after 2015. While these aren’t OPS+ projections, Baseball Prospectus’s PECOTA projections (from before the season) suggest that both Donaldson and Tulowitzki should clear 115 easily. Using BP’s holistic batting metric True Average, Donaldson is projected to hit .290, .281, and .280 through the remainder of his years under Toronto’s control. In that same time span, BP projects Tulowitzki’s True Average to be .310, .308, and .307.

Not only that, but the 115 OPS+ criteria I used above probably undershoots what Donaldson and Tulowitzki, together, are capable of. If we raise it to 130, there are only six seasons since 1961 where left side infield teammates performed to such a level. If we move it to 140—around where both players have been since 2013—we get only one season, Ramirez and Cabrera in 2007.

Troy Tulowitzki’s admiration of Derek Jeter is well known. Indeed, while he was weighed down with trade rumors, that admiration intersected with the demand for an easy story to create far too many "Tulo to the Yankees…?" type stories. It’s possible that Tulowitzki is closer to his hero while wearing Canadian blue than he would be if he were wearing Yankee pinstripes. The indispensable assist comes from Josh Donaldson on his right. What they’ve done and what they can do indicates that Donaldson and Tulowitzki have a chance to truly be a historically great left side of the infield.

★★★

Stats current through Sunday's games.

Eric Garcia McKinley is a contributor to Beyond the Box Score. He writes about the Rockies for Purple Row, where he is also an editor. It took him exactly seven days to go through the five stages of grief and come to terms with the Tulowitzki trade. You can find him on Twitter @garcia_mckinley.