Howard, who was last with the Atlanta Braves on a minor-league deal, was adamant he wouldn't contemplate retirement after being released in May.
"By no means am I done," Howard said at the time. "My plans haven't changed to try and play. My mindset with that hasn't changed at all."
The three-time All-Star played with the Philadelphia Phillies from 2004-16, slashing .258/.343/.513 in 1,572 major-league games. The Phillies elected to part ways with the former Rookie of the Year and MVP following the 2016 season, however, declining his $23-million club option for 2017.