Castellanos, 27, was traded by the Detroit Tigers to the Chicago Cubs at the 2019 trade deadline. He hit .321/.356/.646 with 16 home runs and 21 doubles in 51 games with the Cubs.
After the Giants non-tendered Kevin Pillar on Monday, the club is projected to roster Mike Yastrzemski, Alex Dickerson, and Steven Duggar as its primary outfielders if San Francisco doesn't make another acquisition.
The Giants will have plenty of competition in their pursuit of the outfielder. The Cubs are reportedly interested in a reunion, and the Cincinnati Reds, Miami Marlins, and Chicago White Sox have all been linked to Castellanos this offseason.
Castellanos is apparently also open to playing either corner infield spot, Morosi adds. While the bulk of his playing time in recent seasons has come in right field, Castellanos came up as a third baseman and played 527 games at the hot corner while a member of the Tigers.