This. With his length youd figure hed be a nightmare in the passing lanes. Defense is all about effort and i have no doubt if he puts in the effort he could be a good defensive player.
Effort is a big part, but I'd hesitate to it's ALL about effort. At the NBA level it's also mental... Making the right calls on screens to your teammate, even remembering what the calls are (they have different words each quarter sometimes for the angle of the ball screen, etc.), whether there will be a roll or pop. There was a really good article on Drummond a couple years ago about how he was having trouble adjusting to this.
Communication is HUGE. and they have to remember which players to step out on, who to go under screens with, what are their shooting spots to deny, etc. NBA level defense is incredibly complex.
Now, if his coach is saying his effort isn't there it's certainly possible. In my mind it is probably a combination of factors. But Lamb is also the kind of guy that is just more laid back. Even in his time at Uconn, it was like he didn't really care. Maybe it's his smooth athleticism instead of explosive, or his demeanor. But at the same time he would stuff a dunk in that Columbia players face..
Sorryy... being a homer, I want the guy to succeed.