Cruz apparently injured himself while swinging at a 2–2 pitch from Cleveland Indians starter Corey Kluber. Mariners' manager Lloyd McClendon came out of the dugout to check on Cruz, and quickly guided Cruz back toward the dugout.
Dustin Ackley came in to replace Cruz and worked a walk.
The news is a potentially devastating blow for a Mariners team that recently lost nine games during an 11-game homestand in which it failed to score more than three runs in a single game.
Cruz not only lead the American League in home runs but was also fourth in batting average at .326, behind Prince Fielder, Jason Kipnis and Miguel Cabrera.
Cruz only had one extra-base hit in his past 11 games, after starting the season especially hot.
You can watch video of the at bat here.