The Cubs' shortstop went 0-for-5 against the Brewers, snapping his streak of 43 consecutive games having reached base safely, dating back to Aug. 15.
"It was amazing," Castro said. "I have to keep going."
Castro had one last chance in the ninth against John Axford with the bases loaded, but the Brewers closer struck him out to end the game.
"He threw me a fastball, and it froze me," Castro said.
Cubs manager Dale Sveum is hoping the 23-year-old shortstop can learn more discipline at the plate.
"It just puts you in a whole other category," Sveum said. "It puts you in situations where you'll get a lot better pitches to hit, and when you have good pitches to hit, you're going to be more successful and more beneficial to the team. ... And you'll hit more home runs. The bottom line is getting a good pitch to hit."
Castro's 43-game streak was the second longest by a Cubs player in the last 94 years, trailing only Riggs Stephenson, who reached in 44 consecutive games in 1928.