No player in the history of the NBA has had a TRB% over 30%. Joel's career best was 22.1% in 2008-09. He led the league in TRB% that season, and had two other seasons of TRB > 20% in his career. TRB% > 20 is indeed elite, and his career TRB% of 18.6 is definitely above average.
Lopez does have unimpressive individual rebounding stats, but his teammates rebound better when he is on the floor. Aldridge got off to a slow start, but has had 4 straight double doubles, is averaging 12.5 reb over the last 4 games and has now matched his career best of 9.1 rpg. Batum (6.4 rpg, previous career high 5.6 rpg), Matthews (5.0 rpg, previous career high 3.4 rpg) and Lillard (4.8 rpg, previous career high 2.9 rpg) are all averaging career highs in rpg, by a significant margin playing with Lopez. So, all 4 other starters are at, or significantly above their career best rpg, and the Blazers are out rebounding their opponents by 4.4 rpg (44.9 vs. 40.5). They are also 7-2.
So, would Asik really help the team rebounding that much, or would he just be getting rebounds at the expense of his teammates? In spite of Asik's superior individual rebounding stats, I'd be inclined to stay with what is working for the team. Why fix what isn't broken - and risk screwing up the team chemistry in the process?
BNM