That's good news for the A's, who watched Alonso drop to a heap in agony. He stayed on the ground for some time, before being escorted to the dugout with a trainer by his side.
"Just got me in a good spot," Alonso said. "I should be fine. I don't think it's anything major, so we'll just have to see how I wake up tomorrow, and get some treatment on it. I feel like I've taken some foul balls before, this one just got me in a good spot."
"My guess is he's going to be pretty sore tomorrow," manager Bob Melvin said.
Pinch-hitter Rajai Davis assumed a 2-2 count against Astros reliever Luke Gregerson and struck out swinging. Alonso was credited with the strikeout.
"I'm walking OK," Alonso said. "A little tender, but time will heal, and it should be fine."
Alonso, who has a career-high 17 home runs and is the club's leading candidate to play in the MLB All-Star Game presented by Mastercard, is not scheduled for X-rays.
"Any time someone goes down like that, and when it's off the knee, that means it's all bone, so certainly your mind goes somewhere you don't want it to," Melvin said. "But he recovered a little bit afterward, and was walking around some, so I haven't heard anything about an X-ray at this point."
Jane Lee has covered the A's for MLB.com since 2010.