Gray has only thrown 3 1/3 innings since Sept. 28, so he's scheduled to throw a simulated game Thursday at Minute Maid Park.
"He's had some really big starts, his last two starts have been outstanding," Yankees manager Joe Girardi said of Tanaka. "And there's a lot of times that he really gets on a roll and he puts a bunch of them together and we're going to need a good one from him tomorrow."
The Yankees made the announcements Thursday afternoon, less than 24 hours after they knocked off the Cleveland Indians in the decisive Game 5 of the AL Division Series.
Tanaka pitched Game 3 of that series, striking out seven, walking one and allowing three hits in a 1-0 victory in New York.
Tanaka will oppose Astros left-hander Dallas Keuchel in the series opener. The two also met in the 2015 AL wild-card game, with Keuchel and the Astros winning 3-0.
"We're well aware he's had success against the Yankees, we're very confident in this ballpark. He's pitched extraordinarily," Astros manager A.J. Hinch said of Keuchel.
Justin Verlander will take the mound for the Astros in Game 2.
"The fact that we have both [Verlander and Keuchel] is a huge advantage for us," Hinch said.