Jason Castro made his MLB debut on Tuesday. Manager Brad Mills had three rookies in the starting lineup Tuesday and figures to keep them there awhile. Righthander Jordan Lyles, who was the Astros' second pick in 2008, may make his major league debut sometime this summer, and by next spring, outfielders Jack Shuck, Brian Bogusevic and Jay Austin could be competing for jobs. During the Astros' 3-1 loss to the Giants on Tuesday night at Minute Maid Park, five of the eight position players were 27 or under, and these last 92 games will be invaluable playing time for the rookies ? Castro, third baseman Chris Johnson and shortstop Tommy Manzella.