From the Los Angeles Times:

Shortly before Saturday's exhibition against Phoenix, Clippers Coach Mike Dunleavy pulled Thornton aside.

The message?

More versatility equals more minutes.

"He played the majority of his minutes at power forward and he learned the plays right before the game," Dunleavy said.

"I knew Tim wasn't going to be here and I said, 'Look, I'm not trying to mess you up at all and if it's too heavy for you, no problems. But do you think you can run these plays?'"

"I gave him about five plays and asked if he could handle that and he did and he played pretty good. I was pretty pleased by that."

Thornton was trailing only the Seattle SuperSonics' Kevin Durant among rookie scorers entering Sunday's game at a 17.6 per game clip. The number also leads all Clippers scorers.

The offense is there, but learning the defensive schemes, especially for two positions, may take a while.

"Coming in here, I knew I was going to be playing" small and power forward, Thornton said. "I'm still stressing on some of the rotations." [emphasis added]

 
This is a key point and one of the decisive factors in whether Al Thornton will be a useful NBA forward.  Think about Shawn Marion for a second and how he can anchor PHX’s defense – which is much better than it’s given credit for -- whether he’s playing the small or power forward position.  Marion and Thornton are the same size and both display uncanny quickness.  Marion is often outsized at the 4 defensively, but he’s learned to leverage his quickness – both in establishing himself defensively on the block and in help situations on the rotation. 

I don’t mean to suggest that Al Thornton can be Shawn Marion, but the modern day Western Conference forward – unless he’s a post specialist – has to know how to function at the 4 on both ends if called upon.