The move, retroactive to Aug. 2, was announced before Friday afternoon's game against the Washington Nationals.
"I was not aware of it prior to that. I guess it's been ongoing and he has probably kept it kind of quiet," Maddon told reporters Friday.
Russell, 23, is batting .241 with 20 doubles, 10 home runs and 36 RBIs in 97 games with the Cubs this season.
In a corresponding move, the Cubs called up left-hander Rob Zastryzny from Triple-A Iowa.
Zastryzny, 25, joins the Cubs for the second time this season after he made one appearance during a brief stint in May, giving up three runs over four innings. He is 1-2 with one save and a 6.67 ERA over 27 innings in 10 appearances (three starts) with Iowa this season.
Zastryzny spent two months on the disabled list due to a lat injury and made two starts with Iowa since being activated on July 25. He made his big-league debut with the Cubs last season, going 1-0 with a 1.13 ERA over 16 innings in eight appearances (one start).