Johnson, Manzella and right-hander Billy Sadler will join the team Tuesday and be in uniform when the Astros open a three-game series against Atlanta at Minute Maid Park. Manzella was already on the 40-man roster, but Houston had to purchase the contracts of Johnson and Sadler.
"This is a situation for them to come and learn from some of the best players that ever played, particularly in this town," Astros manager Cecil Cooper said. "I think it's good for Tommy and Chris to seem how [Geoff] Blum, [Miguel] Tejada and Lance [Berkman] go about their work.
"Just to get a chance to watch some real good veteran players prepare to play is a good experience for them because they're the future. They're the kids we're counting on down the road."
Cooper said he met with Tejada and told him he would lose some playing time to Manzella, a third-round Draft pick in 2005 out of Tulane. He plans to have similar conversations with Blum and Aaron Boone concerning playing time for Johnson, a fourth-round Draft pick in '06 out of Stetson.
"I'm going to explain to them that these kids need some exposure and it might cost a little bit of playing time," Cooper said. "It won't be a lot, but they will have a chance to get their feet wet."
Cooper admitted he had to convince general manager Ed Wade to call up Johnson, who wasn't on the 40-man roster.
"Tommy was the one we had discussed about possibly bringing up, and we were both in [agreement], and Chris was the last one," Cooper said. "That one I kind of pushed a little bit. It's going to be good to have them with us."
To make room for Johnson on the 40-man roster, the team transferred left-hander Mike Hampton to the 60-day disabled list from the 15-day disabled list.
Johnson, 24, hit .281 with 13 home runs, 20 doubles and 42 RBIs in 104 games for the Express this year and was named Round Rock's Offensive Player of the Month for August after hitting .333 with seven homers and 20 RBIs.
Manzella, 26, hit .289 with nine home runs, 31 doubles and 56 RBIs in 133 games at Round Rock. He was named by Baseball America as the Astros' best defensive infielder the past three seasons.
Sadler, 27, went a combined 5-3 with a 5.12 ERA in 16 starts at Round Rock and Triple-A Fresno. He was signed by the Astros to a Minor League contract Aug. 6 after being released by the Giants, the team for which he appeared in 38 Major League games in 2006 and '08.
Brian McTaggart is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.