According to this data, he's the second best manager in the major league in bullpen management (Gibbons is 2nd worst)
http://grantland.com/the-triangle/2015-mlb-playoffs-bullpen-managers-mike-matheny-joe-girardi/
The Yankees are also firmly in the playoff race despite a roster that...should not be anywhere near it.
I'll quote a bit more from that article:
"Girardi is already regarded as a bullpen whisperer; BMAR just clinches the case. In April 2014, Fox Sports reporter Ken Rosenthal passed on the praise of a “rival AL exec†who’d told him that Girardi was “the best [he’d] seen at managing a bullpen.†The BBWAA is onboard: Girardi (who won the NL Manager of the Year Award in 2006) hasn’t finished lower than sixth in AL MotY voting since 2008, even though his high-payroll teams have missed the playoffs twice. After eight seasons in the country’s largest media market, Girardi’s bullpen use still gets a seal of approval from both bloggers and tabloid beat writers, two tough-to-please constituencies. I
t’s even more impressive that Girardi leads in BMAR given his reluctance to use relievers three days in a row, which limits his options in some high-leverage situations: This season, the Yankees ranked 29th in reliever appearances on zero days’ rest, but they also ranked first in multi-inning outings. For what it’s worth, the Yankees have exceeded their Pythagorean win-loss record by an average of two games per season during Girardi’s years at the helm, as have the Angels during Scioscia’s 16 years in Anaheim."