In a three-run fourth inning for Cincinnati, Phillips advanced from first base to third base on Jay Bruce's RBI single through the left side. As Phillips approached third base, where no one was covering, Castro backed into him and fell on top of him while trying to make a tag.
"It was kind of a foot race to third base. I think Castro took the throw on the run, and he and Brandon both went down," Reds manager Bryan Price said. "Somehow, Castro came down on Brandon and then Brandon had a whiplash type of event, I guess."
Phillips was slow to get up as Bruce was thrown out trying to take second base after the collision. Reds head trainer Paul Lessard and Price spent several minutes checking on Phillips, but he remained in the game.
In the bottom of the fourth, Phillips made a diving stop on Dexter Fowler's grounder. After the top of the fifth, Phillips was replaced by Skip Schumaker.
Phillips passed a concussion test after he departed the game.
"We think he might be OK by [Wednesday] to play," Price said.
Although he lacks an extra-base hit, Phillips is batting .300 this season after a 1-for-2 night. He had four singles in Monday's loss to the Cubs and is 7-for-13 over his last three games.
Mark Sheldon is a reporter for MLB.com. Read his blog, Mark My Word, and follow him on Twitter @m_sheldon. This story was not subject to the approval of Major League Baseball or its clubs.