Johnson was hitting .245 with six homers and 36 RBIs after a breakout rookie season a year ago when he hit .308 with 11 homers and 52 RBIs in 94 games. He was batting .267 after a miserable April this year in which he batted .185 to start the season.
Johnson feels the Astros have lost confidence in him.
"It's tough, especially the way I started and battled back," he said. "It's tough to know that people lost confidence in you. That's the hard part, the people don't believe in you anymore. Our goal is to go down and build that confidence back up to show guys I can play."
Wallace, acquired a year ago in a trade with the Blue Jays, hit .388 in April and was among National League leaders in batting average the first 2 ½ months of the season. But he's batted .160 since June 18 and is currently in an 0-for-18 drought. He has 26 RBIs in 317 at-bats and recently saw his playing time dwindle.
"I think with the way things have been going, especially the last few days, trading away some of the guys to get younger, I think we thought it was a situation they were going to continue to build with a young core and we were hoping to be a part of that," Wallace said via phone upon the team's arrival in Houston. "It's definitely surprising. We'll go down there and take care of business."
Combined with the trades of Pence, Bourn and Jeff Keppinger, the Astros have seen five starters depart their 25-man roster in less than two weeks.
"The players who sustain success and longevity at the big-league level are the ones who have shown to be capable of making adjustments," Astros general manager Ed Wade said. "Both Chris and Brett have worked hard to try to make adjustments and we know what they're capable of doing and we just have to try to do something different."
Paredes, acquired from the Yankees a year ago in the trade for Lance Berkman, is the third player the Astros have called up from Double-A in the past two weeks, joining second baseman Jose Altuve and outfielder J.D. Martinez.
The switch-hitting infielder was hitting .270 with the Hooks with 10 homers and 41 RBIs and 29 steals in 93 games this season, but he has 84 strikeouts and 15 walks. He recently made the move to third base from second base.
Bogusevic, 27, rejoins the Astros for his third stint in Houston this season. The left-handed-hitting outfielder/first baseman is batting .194 in 39 games with the Astros in 2011.
"Jimmy moved over from second base to third base a couple of months ago and has done a good job there," Wade said. "We're not putting high expectations on Jimmy at this point. He's still a developing player, and we really like the athleticism he's shown and we think he's going to be a good, solid big-leaguer.
"Clearly we're acknowledging it's a little before his time, but we're going to go ahead and give him a chance to get out there and get his feet on the ground and see how it works out."
Paredes will be the seventh Astros player to make his Major League debut this year and third in less than two weeks. Altuve was called up from Double-A on July 19, and Martinez made his Major League debut Saturday following the Pence trade.
Pitchers David Carpenter, Jordan Lyles, Aneury Rodriguez and Jose Valdez have also debuted this year.