Chicago first baseman Daryle Ward also departed after straining his right calf muscle while trying to catch a popup in the second inning.
Floyd collided with Giants pitcher Matt Morris, who was covering first in an attempt to turn a double play on Floyd's grounder.
After running into Morris, Floyd tumbled and fell to the ground. He stayed in the game and later scored all the way from second on a passed ball by Bengie Molina.
But he was replaced to start the fourth by Angel Pagan.
Floyd had just returned to the Cubs' lineup yesterday after missing three games with a sore left shoulder.