It happened in the bottom of the sixth inning. With two outs, Asdrubal Cabrera singled. Carlos Santana lifted a high pop that finally came down near the first-base dugout. Both Fielder and Avila converged, but neither gave way and Fielder's elbow collided flush with Avila's jaw. The ball dropped to the ground, followed shortly by Avila. He stayed there for a few minutes, before finally standing up and walking off the field under his own power.
After a delay, during which Gerald Laird suited up and entered as Avila's replacement, Cabrera stole second base, and then Santana hit a ground ball toward the hole between third base and shortstop; Miguel Cabrera snagged the grounder, then threw high to Fielder at first base. Fielder tried the swipe tag on Santana and replays suggested he was successful, but the umpire called Santana safe and Cabrera sprinted home to make the score 5-3.
This was a big game, as the Tigers needed a win to remain just one game behind the first-place White Sox in the American League Central.
Update: According to the Tigers, Avila was showing no concussion symptoms, his playing status day to day.