Mejia ninth inning on Sunday.
Castro nonetheless produced an opposite-field solo homer on the first offering from Mejia in the frame to lift the Cubs to a 2-1 win against the Mets at Citi Field.
It was the first home run surrendered by Mejia since June 1 in Philadelphia -- a span of 27 games and 27 1/3 innings.
Opponents are now hitting .345 (10-for-29) on first pitches this season against Mejia.
“I tried to throw outside. That was outside,” said Mejia, who reiterated his mechanics are unaltered because of his hernia. “He got it. ... I wanted to get ahead. I tried to throw the first pitch for a strike. In that situation, I don’t want to throw the first pitch for a ball. ... I think he was waiting for a fastball, because I threw a fastball away and he swung.”