We knew it wouldn't be easy.

Replacing a living legend as the New York Yankees' closer would be difficult to do for any pitcher in baseball.

Relief ace David Robertson has earned the unenviable task of attempting to do so, after becoming one of the game's most-feared relievers in the last year or two.

Since 1997, Mariano Rivera has finished games for the Yankees, making it look effortless with a simple approach and an understated demeanor. The man is as respected as anyone in the league, for his grace and humility as much as the ruthless dominance he has displayed throughout his career.

When the presumed greatest closer of all-time collapsed in a heap on the warning track of Kansas City's Kaufman Stadium, there was a collective gasp heard around the league. 

Speculation was rampant immediately. Was this the end? Had we just witnessed the tragic conclusion of one of the greatest pitching careers in baseball history?

If you watched any baseball coverage shortly following the Yankees' game in Kansas City that evening, you might have surmised that Mo had not only suffered a serious injury, but had perhaps passed away. The obituaries came fast and furiously along with career retrospectives and heart-felt tributes to the man.

Such is the influence of Mariano Rivera.

Unfortunately, once he was examined by the Royals' team doctors and then at a local hospital, the Yankees' worst fears were realized. Rivera's ACL was torn, along with his meniscus, effectively ending his season and potentially threatening the career of the 42-year-old hurler.

The loss of MLB's all-time saves leader suddenly created a massive void at the end of the Yankee bullpen.

Thankfully, the club had populated their relief ranks with talented arms capable of stepping into the role that had been occupied by Mo for a decade and a half.

Prior to 2011, the Yankees had signed Rafael Soriano to a massive contract, paying the one-time elite closer $35 million over three seasons to serve as a set-up bridge to Mariano. Soriano didn't take to the role immediately and suffered through a mid-season injury battle which caused him to miss much of May and all of June and July. 

Soriano was often assumed to be the heir apparent whenever Mo decided upon retirement.

However, there was another contender for the role, right-handed escape artist David Robertson. 

The 27-year-old reliever began attracting some attention in 2009-10, when he posted solid performances highlighted by stellar strikeout rates.

In 2011 though, Robertson took an immense leap forward with an amazing season that saw him earn his first All-Star berth. Over 66.2 innings, he went 4-0 with a 1.08 ERA, while striking out 100 opposing hitters. He allowed only 55.4 hits per nine innings and one solitary home run all season. 

Suddenly, the Alabama native was becoming renowned for his "sneaky-fast" fastball, a wicked curve and an uncanny penchant for escaping difficult jams that would overwhelm lesser pitchers.

Robertson continued his dominance into 2012, serving as one of the primary set-up men for Mariano. Over his first 12 appearances, Robertson allowed only seven hits, no runs and owned an incredible strikeout-to-walk ratio of 21:3. 

His run of shear dominance was enough for the Yankees to bypass the closing experience of Soriano in favor of Robertson.

With the evolution of his pitching role, suddenly Robertson isn't finding things to be as easy as they had been for him.

Of course, he has only been presented with two save opportunities since Mo went down, so this isn't necessarily an indictment of his ability to close games.

However, it must be noted that after allowing only 10 base-runners over his first 12 innings of relief, Robertson has now allowed seven opponents to reach base in only 1.2 innings during those save opportunities. 

He successfully converted his first chance on Tuesday against Tampa Bay, but he walked a tightrope the entire time and was fortunate to escape with the save. After he loaded the bases on a single and two walks, Robertson struck out Carlos Pena looking to end the threat.

Handed another save opportunity the next evening, again against the Rays, Robertson imploded. With only a 1-0 lead, he allowed consecutive singles to begin the inning, before walking Ben Zobrist to load the bases with no outs. He again struck out Pena looking with the bases full, but then allowed a game-tying sacrifice fly to B.J. Upton. 

Disaster struck next, as Robertson got ahead of Matt Joyce on consecutive strikes, but couldn't put him away and served up three-run home run to the Tampa outfielder on a 1-2 pitch.

The lead was gone and the Yankees were unable to dig out of the 4-1 hole, losing a game that they had led since Robinson Cano gave them a 1-0 lead with a first inning double.

Two games is clearly not a disaster and Robertson could potentially embark upon another dominant run, asserting himself as the Yankees' closer.

However, with two poor outings in a row and an experienced closer looming in the bullpen, Robertson's leash may not be long.

He was after all, one of the game's elite set-up men and Soriano might have been the league's best closer as recently as 2010. The fact that he pitched in the AL East that year, as a member of the Rays, may bode well for his ability to handle the division. 

For now, the Yankees are professing their faith in Robertson, but if he doesn't soon revert to his scorching form, the club may opt to explore their other available options.