Atchison, who gave up a two-run homer to Matt Holliday in yesterday's game, has been pitching with a tear in his elbow for years. The 37-year-old has been mostly ineffective with the Mets this season, pitching to a 4.50 ERA and 1.44 WHIP in 18 innings, allowing 20 hits and six walks while recording only six strikeouts.
McHugh has been pitching very well in Triple-A this season, posting a 2.74 ERA in 49 innings with a 36/9 K/BB ratio and only two HRs allowed. He made his MLB debut last August 23 when he threw seven shutout innings against the Colorado Rockies with nine strikeouts, after which things went south for him.
It's unclear what kind of role McHugh would have on the roster, but our best guess is that he would function as a long man.