Hamilton, 29, did not appear in a game for the Giants this season after signing a minor-league deal with them in February. He had been practicing at the team's alternate training site after missing most of summer camp for an undisclosed reason.
The speedy outfielder has recorded 73 defensive runs saved and 299 steals during his seven seasons in the majors. At the plate, the Mississippi native owns a career slash line of .242/.297/.326.
Humphreys has never appeared in a big-league game. The 24-year-old owned a 2.60 ERA and 1.03 WHIP in the minors but never pitched above A-ball. He's missed most of the last two seasons due to injuries.
Humphreys was designated for assignment by the Mets on July 29.