Hoerner and Happ were both attempting to catch a shallow fly ball to center. While Happ returned to his feet, Cubs trainers helped place him onto a cart.
NBC Sports Chicago's Gordon Wittenmyer noted that after the collision Happ had a bloody nose and "did not appear to suffer any injuries to his extremities."
Cubs manager David Ross addressed the situation following his team's 13-12 defeat.
"I checked in," Ross told reporters. "I talked to Ian. He feels fine. Pretty good blow to the face. I haven't heard the doctor's report yet, but I did talk to Ian and he's in good spirits."
Happ went 3-for-5 at the plate with three RBI and three runs scored. As part of a four-run eighth inning for Chicago, he homered to right to tie the score at 12 runs apiece.
The 26-year-old has struggled to open the 2021 season. Entering Sunday, he was batting .139 with 32 strikeouts in 97 plate appearances.
Chicago has already been without Joc Pederson since a 3-1 win over the New York Mets on April 20 because of a wrist injury, and right thigh tightness kept Contreras on the bench Sunday before Happ's exit required him to enter the game.
While Happ hasn't performed up to expectations, missing him for an extended stretch would be another setback for the Cubs, who are five games back of the first-place Milwaukee Brewers in the National League Central.