The Giants designated Brennan for assignment Wednesday.
Brennan will give up his spot on the 40-man roster to left-hander Reiver Sanmartin (hip), who was reinstated from the 60-day injured list. The 28-year-old Brennan had produced a healthy .869 OPS over 117 plate appearances at Triple-A Sacramento in 2026, but he's had far less success over parts of five major-league seasons between stops with the Guardians and Giants, slashing just .263/.301/.365 (86 wRC+).