Prior had been pitching for Triple-A Pawtucket since May, but he was released on Friday.
The PawSox needed to clear a roster spot for Pedro Beato, the player to be named from the Mets in the Kelly Shoppach trade.
In 19 games with Pawtucket, Prior was 1-0 with a 3.96 ERA. Over 25 innings, he had 38 strikeouts while holding opponents to a .172 average. However, he had 23 walks.