Fields was sent down in a roster crunch on Aug. 20 despite going 4-1 with a 2.20 ERA in 43 games. Villar, who will provide the Astros with speed off the bench, hit .263 with four stolen bases in 38 games with the Astros earlier this year. Thatcher was designated for assignment by the Astros in July after going 1-3 with a 3.79 ERA in 36 games this year.
"Guys will trickle in as the week comes on throughout this homestand," Astros manager A.J. Hinch said. "We're going to use the first part of September very similar to how you use it during the season. If you have a game where we need something for the next day, we'll bring somebody up. We'd like our guys to stay sharp by playing. All of our affiliates are in the playoffs, and so having the guys at Triple-A and Double-A stay sharp by playing in games rather than come up here and sitting on the bench the first few days, we'd rather have a plan."
Outfielder Preston Tucker is expected to join the club Wednesday, which is the first day he can be recalled after getting sent down Aug. 22.
The Astros called up catcher Max Stassi on Saturday when they put Jason Castro on the disabled list with a strained quadriceps. Stassi would have been the third catcher had Castro been healthy, but Hinch said the club plans to stick with just two healthy catchers -- Stassi and Hank Conger -- until Castro returns. Evan Gattis could serve as emergency catcher.
"We'll react accordingly to how Castro's recovery is coming along," Hinch said. "We've got some favorable first couple of days from Castro. I know he's feeling a little bit better, and so if he continues to progress, we won't have a need to call up a third catcher.
"If we feel having Gattis as the emergency catcher is not enough and Castro is not mending fast enough, we would obviously think otherwise. The plan right now is to watch Castro and then read and react as the Double-A and Triple-A playoffs end."
Hinch said he didn't expect to have a long reliever coming up, saying a starter could be placed into the bullpen. A third left-handed reliever is also likely to join the club soon.
Brian McTaggart is a reporter for MLB.com and writes an MLBlog, Tag's Lines. Follow @brianmctaggart on Twitter and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.