I'm confident that Horford will be top 20 next year barring injury.
But depending on who's in your league, you may be able to grab him around 30, and that would be a huge steal.
Look at his rank in 9cat the last 4 years
2015-16: 16th overall
2014-15: 17th overall
2013-14: 14th overall
2012-13: 15th overall
He did have an injury in 2013-14 season in which he sat out most of the season. But he played 76 and 82 games in the 2 years after that so (knock on wood) he should be safe going forward. Boston doesn't really have a star big-man outside of Horford so I think he'll be utilized well and should produce excellent numbers