Celtics' Ainge: Zeller's shoulder caused Horford concussion

The Boston Celtics have finally revealed the cause of Al Horford's mysterious concussion.

Team president and general manager Danny Ainge divulged on The Sports Hub 98.5's "Toucher and Rich" show that the 6-foot-10 forward sustained the injury after colliding with Tyler Zeller's shoulder during practice. He has yet to return to the lineup, missing the Celtics' last eight contests.

Horford has appeared in just three games since signing a four-year, $113-million deal with Boston this summer, averaging 12 points, 5.3 rebounds, and 2.3 blocks in 29.3 minutes per game.

Boston has struggled on defense without the nine-year veteran, surrendering 105.3 points per 100 possessions this season.