Walker, a career .272/.338/.431 hitter, will replace Daniel Murphy as the team's starting second baseman. The 30-year-old, who hits for less contact but with slightly better power and plate discipline, will enter free agency at the end of the 2016 season.
Having spent the past six years in the Mets' rotation, Niese was the club's longest-tenured pitcher. He holds a career 3.91 ERA and 6.96 K/9, but those marks dropped to 4.13 and 5.76, respectively, last year. He provides Pittsburgh with needed rotation depth, but he would have become the odd man out of New York's rotation when Zack Wheeler returns from Tommy John surgery.
While Walker will likely serve as one-year rental, it represents a compromise between spending big on a veteran and awarding the starting gig to the untested Dilson Herrera. It's a logical trade that makes sense for both parties, but one that signals the end for two long-time Mets in Niese and Murphy.