What he's done to get picked up in fantasy leagues:
He's got himself a shiny batting average of .338, some stolen bases and position eligibility at first base which is chronically starved for base stealers. He does have six multi-hit games since the 8th of June.
Is this going to come close to holding up in the long term:
In a word, no well unless you think a .410 BABIP is going to hold up in the long term.. Once that goes, the batting average starts to fade, yeah there will be some stolen bases but unlike most players with 1B, 3B or OF eligibility, there isn't much power for him to fall back on in this scenario and it appears there isn't much defensive strength to fall back on.
Best case scenario is he keeps his batting average in the .260-.280 range, manages to steal 15-20 bases and pops maybe a handful of home runs. Good enough to stay in the majors for awhile? Sure Good enough to regularly hold a starting job once the Red Sox have a season where they are major contenders? Probably not.
Has he turned a small corner without it being blatantly obvious? Possible but not likely. Compared to 2012/2013 he has hit slightly more line drives and most importantly has continued to decrease the ground balls being hit. My instinct thinks that a best case scenario may be him turning out to be a James Loney caliber player most likely scenario is that he becomes a journeyman utility infielder.
When he got traded along with Joel Hanrahan to the Red Sox, it was likely more as a throw in but even the modest sample size of success he has had over the last month has been more than what they got out of Hanrahan (thought the Red Sox are still on the short side of this trade).
He may not become a star but for being a 9th round pick in the 2009 MLB draft, he's still turned out pretty good as he is one of six players from that round year in the MLB and amongst those only Aaron Loup and Chase Anderson have a higher WAR.
All numbers, etc referenced found on either fangraphs.com or baseball-reference.com