Even a 10-10 night from the field wasn't enough for Pau Gasol to surpass Al Horford in PER. Horford's 2010-11 performance trails only Chris Paul.
Now, PER is an efficiency stat so it may overrate Horford as neither his playing time nor his usage rate is typical of a contending league-best player. To that end, John Hollinger has another stat, EWA (Estimated Wins Added). By that measure, Horford has only been the (joint) third-best player in the league, providing production equal to LeBron James (16% of the way into the season) despite playing 85% of the minutes James plays and posting a usage rate more than one-third lower.
Consider this today's attempt at emphasizing how well Al Horford is playing.