umm, false.
per synergy, on all plays where al is the primary defender, the jazz give up 92 points per 100 possessions. when it's paul or derrick, that number falls to 83. kanter is at 77. sap is far better defending the roll man on the PnR, isos AND spot-up shots (where al is truly awful, allowing 123 pts for every 100 spot-ups by his man). the only frequent defensive play where al is empirically defending well is post-ups, where all 4 jazz bigs are between 79 and 85 pp100, with al leading the way and paul right behind him at 81.
unfortunately, synergy doesn't keep numbers on help defense because they can't possibly automate a system that takes into account 30 teams' defensive philosophies on who should rotate when and assess the possession -- but just from watching the sheer number of times al sees a guy coming down the lane and moves to the side, i can tell you it's pretty bad, too.