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Since Al Horford was on the Atlanta Hawks he wanted to be a power forward and not a center. When he signed a max contract with the Celtics, Horford reiterated that wish. The Celtics tried at times to play a center next to Horford, but the team always played better when he was the lone big on the floor.
Fast forward to this past summer when Horford chose to leave the Celtics for more money and what he thought was a starting power forward role on the 76ers. Sounded good to him and if this was an older era of the NBA it likely would have worked.