5:14pm: John Hickey of the Seattle Post-Intelligencer reports that the Mariners have designated infielder Tug Hulett for assignment to make room for Pena on the 40-man roster.
Pena, 26, was designated for assignment when the Brewers claimed Nick Green from the Angels. Brewers general manager Doug Melvin explains:
"It’s like trading Pena for Green," said general manager Doug Melvin. "We needed starters. You hate to give up on a good arm but that was our need."