The Yankees brought Jacoby Ellsbury in to be their leadoff hitter. They knew that, he knew that, and Brett Gardner knew that. With Ellsbury hitting first and Derek Jeter holding down the #2 spot based more on reputation and tribute rather than merit, Gardner was left without a clearly defined role or spot in the batting order heading into the season. Best suited to hit in the 1 or 2 spot, Gardner was relegated to the bottom third of the order as a pseudo-sorta second leadoff hitter.
At least he was until the injury problems started. Between Mark Teixeira's strained hammy and now Carlos Beltran's elbow bone spur, the Yankees have had to use Ellsbury to lengthen the middle of their lineup a lot in the first 40+ games. When Joe has had to bump him down to the 3-hole, it's been Gardner who's gotten the call to replace him atop the order. While Gardner's first go-round in the leadoff spot in early April wasn't bad by any means (15-48, 4 R, 2 BB, 9 K, 1 SB in 10 G), his second effort over the last week and change has been much better.
Gardner was reinstated into the leadoff spot on Sunday, May 11th, the final game of the series in Milwaukee. In the 8 games he's played there since, he's gone 11-32 (.344) with 5 XBH, 7 R, 7 RBI, 5 BB, 9 K, and 3 SB. He's been hitting for the entire month (.323/.400/.516 in 71 PA), that's nothing new. What is new is the diversity Gardner has brought back to his offensive game. Early on this year he was hitting a lot of singles, not drawing as many walks as he usually does, and swinging and missing a lot. The strikeouts are still there in May (26.8% K rate), but the walks are back up (11.3% BB rate) and Gardner has been driving the ball much better. He's being more patient at the plate hitting first, working counts to get himself into something favorable, driving the ball with power when he gets what he's looking for, and taking walks to get on base and create SB opportunities when he doesn't.
The Gardner we've seen over the last 8 games has been a hybrid combination of 2010 Gardner (high BB rate, low power, lots of SB chances) and 2013 Gardner (lower BB rate, swinging for more power, fewer SB chances), and he's been the most consistent part of the offense. Whether he hits his way on or not, Gardner has found a way to get on base 2 or more times in 7 straight games. That's exactly what you're looking for from your leadoff man and Gardner is playing that role incredibly well right now. Him keeping this up should help lessen the blow of losing Beltran for a few months.