KISSIMMEE, Fla. -- Chris Johnson is considerably closer to the big leagues than he was on Tuesday night, but the young third baseman was hardly rejoicing before Wednesday's game between the Astros and Yankees.

"This is not a good day," Johnson said.

He was reacting to the news that Aaron Boone was done for the season, and perhaps for his career, because of a heart condition. Boone and Geoff Blum were expected to platoon at third base for the Astros this season, with Johnson, perceived by some as the club's top prospect among position players, hoping for a midseason call up or, at the very least, a real shot to make the Opening Day roster in 2010.

With Boone out of the picture, Johnson will receive a longer look at third base, although it's still unclear where he fits into the final picture. For now, the incumbent third baseman is Blum, with Johnson still knocking on the door. Will the Astros want Johnson, whom manager Cecil Cooper said will someday be "a big-time player for us," to platoon with Blum, essentially making him a part-time player? Highly unlikely.

The Astros, still absorbing the Boone news, have not yet devised a Plan B.

"We've got C.J. and Blum over there, and they'll continue to compete," Astros general manager Ed Wade said. "If it seems that we need something else down the road, we'll try to address it at that point."

Wade maintained all offseason that he expects a new crop of players to become available toward the end of Spring Training, when many who signed Minor League deals with out clauses become free agents. The Astros could look there for third base help, or they could also explore the trade route. Moving Miguel Tejada from shortstop to third base does not appear, at this time, to be a realistic option.

For now, the Astros will continue to monitor the final two-plus weeks of the Grapefruit League season.

"We haven't looked at anything right now," Wade said. "We're not prepared to anoint any of the young guys as big leaguers right now. As I've said all winter long, we think -- as we get down to the latter stages of Spring Training -- there might be other players available.

"We'll let the rest of a long spring play out and see where we are."

In the meantime, Cooper has no problem giving Johnson a long look at third.

"Sometimes the infusion of youth is a good thing," Cooper said. "We'll keep running the kid out there and see how it goes. You have to give the kid every opportunity. This is our situation right now. I'm not saying that it's not going to change."

For his part, Johnson expressed sadness to lose Boone as a teammate and a mentor.

"[Boone] and Blum," Johnson said, "we were taking ground balls the very first day we were out there, and [Boone] said, 'Hey why don't you try this?' He didn't even know me.

"He spent a lot of time talking with me and working with me. I'm not thinking of myself right now. I'm worried about him."