The Celtics landed Al Horford, the most coveted free agent this summer not named Kevin Durant. They re-signed head coach Brad Stevens and president of basketball operations Danny Ainge to extensions. The good folks in Vegas have them pegged as just one of a handful of teams that should win at least 50 games this year. By all accounts this was a pretty fruitful and productive offseason for the Celtics. “People have been writing about us; they have been saying we’re in the top 10 in the league or top part of the league,” said co-owner Wyc Grousbeck, who was participating in the Shamrock Foundation’s Ninth Annual Teeing Up for Kids Golf Tournament presented by Optum. He appreciates the praise, but Grousbeck is just as locked in on what’s not being said about the Celtics.