Horford is not as big a rebounder as Howard, partly because he shoots outside part of the time and isn't in position for a rebound. Howard is more of an inside player. Another factor is Horford plays next to Millsap, who is a good rebounder. Horford would probably rebound more on the Celtics, though probably not as much as Howard would. That's why I would probably also sign an inside player like Gasol, Biyombo or Ezeli.