Seager was 3-for-11 in the NLDS and is a .211 (15-for-71) hitter in 19 postseason games. He batted .295 with 22 homers and 77 RBIs in 145 regular-season games.
On Monday, manager Dave Roberts said it was likely Seager would be added to the roster.
"Unless something really unforeseen happens, I don't see how he won't be active for us," Roberts said.
During the NLCS, Charlie Culberson filled in for Seager at shortstop. He may start there when the series shifts to Houston, where Seager can be the designated hitter.
McCarthy was 6-4 with a 3.98 ERA in 19 games (16 starts) during the regular season. He only pitched six innings since July 20 due to a knee injury.
Granderson was 1-for-15 with eight strikeouts in the first two rounds. He batted .161 (18-for-112) with seven homers and 12 RBIs during the regular season after being acquired from the New York Mets in August.
Farmer was the third-string catcher in the first two rounds and went 0-for-4 in five games. He appeared in 20 regular-season games.
The Astros did not make any changes from their roster in the ALCS against the New York Yankees.