The Giants reinstated Jackson (back) from the 15-day injured list Monday.
Jackson was placed on the 15-day IL after sustaining a lower-back strain in the Giants' season opener against the Padres. Jackson appeared in 33 games out of the bullpen last season, posting a 2.97 ERA, 1.23 WHIP and 43:15 K:BB over 33.1 innings. The Giants optioned right-hander Nick Avila to Triple-A Sacramento in a corresponding move.