Pau Gasol is not a good defender. That is not a controversial statement.
Pau Gasol is not giving enough effort on defense so far this year. That is a different statement altogether, and unfortunately: also true.
Pau is never going to be winning DPOY or shifting around his feet to stop guards from driving, and that's fine. Expecting a 35 year old center with over 35,000 career minutes to do that isn't fair. That doesn't mean he still can't do more on the defensive end though, especially in regards to getting a hand up to contest shots.
Ricky Rubio targeted Gasol on pick and roll last night and got whatever he wanted. When guards like Rubio go at Pau, it would be so easy for him to simply lift his hand and extend his 7' 4" wingspan to make shots tougher. He has failed to do this over and over this season - his hands stayed glued to his sides:
Pau did this repeatedly throughout this game, and this has been one of his biggest problems defensively throughout the first 7 games.
Gasol tends to cheat a ton and sag way off his man to prevent guard penetration on pick and roll. Not only does he completely ignore guarding his own man, but he is still useless in defending the guard on pick and roll because he won't put a hand up to contest shots:
The best shots in the NBA are open shots. A wide open long 2 jumper is generally a more efficient shot than a closely contested corner 3, despite analytics' general hatred of the former and love of the latter. All Gasol needs to do is put a hand straight up to contest shots like these and the shooting percentages on those shots will drop dramatically. Estimates vary, but there are studies out there suggesting that the simple act of raising a hand up can drop shooting percentages by about 6%, if not more.
This is one area where Gasol should not get a pass. He needs to care a teeny bit. He is physically capable of doing more on the defensive end, and situations like these are a great example.
Gasol has done a passable job of rim protecting this year - opponents improve their FG% by about 5% when Gasol is guarding them on shots within 6 feet which is not good but not horrible either. He's also come up with some big defensive plays like his game-saving block against LeBron in the season opener. But he must do a better job of contesting these short jumpers on pick and rolls for the Bulls' defense to be championship-caliber.