Johnson was dazed when he was struck in the head by a fastball thrown by teammate Yorman Bazardo during a simulated game Tuesday afternoon at Minute Maid Park, but he didn't let the incident ruin his fun.
"That's why we wear helmets," Johnson said.
Johnson, a third baseman, and fellow prospect Tommy Manzella, a shortstop, and right-hander Billy Sadler joined the Astros on Tuesday. Sadler has appeared in a handful of games with the San Francisco Giants, but Manzella and Johnson are awaiting their Major League debuts.
"It's going to be a pretty special moment to put on those pinstripes," Manzella said. "That's going to be a good feeling. It's something I've looked forward to and envisioned since I took batting practice here in 2005, when I signed.
"That was a starting point, but it's where I want to finish my journey. This is where I want to be. I'm going to work hard and do as much as I can and learn as much as I can."
Astros general manager Ed Wade wouldn't say how much the two would play, but he said the experience would be invaluable.
"If nothing else, it's a chance for them to absorb some of the atmosphere," Wade said. "It's a chance for guys to become familiar with the big league surroundings. I'm sure [manager Cecil Cooper] will find some playing time for them along the way, but by and large it's just sort of to absorb what this is all about and hopefully get a clearer understanding of what it means to be a big league ballplayer."
Johnson, 24, and Manzella, 26, were informed of their callups by manager Marc Bombard at Triple-A Round Rock, which had its season end Monday. Bombard brought them both into the dugout after the game, congratulated them on a good season and said they were going to Houston.
"You have a pretty good idea the whole week leading up, but until the words come out of the mouth, you really don't believe it," Manzella said. "It was definitely a humbling experience."
Johnson, the son of former big leaguer and current Pawtucket Red Sox manager Ron Johnson, 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.
"They said to just stay ready," said Johnson, who missed a month earlier in the year when he was hit on the hand by a pitch. "[Cooper] said when he calls my name, just be ready to play. So whatever I can do to help the team, that's what I'm going to do. It was tough getting hurt, but that's another one of those things that happens. I got healthy and played the game and had a pretty good year."
Manzella's road to the Majors has been a tough one. A native of the New Orleans area, he had his boyhood home flooded by Hurricane Katrina shortly after he was drafted out of Tulane in 2005. His mother, Carol, died last year from ovarian cancer.
"You're not thankful for anything like that to happen, but it definitely makes you a stronger person, which I think has made me a stronger player," Manzella said. "Everything happens for a reason. I'm a really big believer in that, and the first thing I thought about when I was told [I was being called up] was how happy my mom would be.
"When I called my dad, that's the first thing he said: 'Your mom would be so proud of you.' Just to hear that, that's when it kind of sunk in."
Manzella 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.
"Playing time is not something I'm focused on now," he said. "The main thing I'm going to do is keep my eyes open and watch how [Miguel Tejada] goes about his business and just try to learn as much as I can and be able to put that into my game."
Sadler, 27, went a combined 5-3 with a 5.12 ERA in 16 starts combined at Round Rock and Triple-A Fresno. He was signed by the Astros to a Minor League contract on 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.