Indiana Pacers recorded a 125-116 victory on Saturday, the fourth time in six games the Nets have conceded 125 points or more, as Nash's side dropped to 1-5 this season.
Brooklyn host the Pacers again on Monday and Nash outlined his frustrations with the Nets as he urged his team to show more commitment.
"It was a disaster," coach Nash said. "How else do you say it? I didn't see the will, didn't see the desire, or the connectivity necessary to get stops and get rebounds.
"We just got to make a bigger commitment and it's got to mean more and we got to care more.
"We have to look deep inside ourselves and what we want to do. What do we want to accomplish? Do we want to give up on this because it's been difficult early or do we want to stay the course and start to build something?
"We've had a lot of really good days here early in the season and we've lost a couple of games; it shakes our mentality, it has shaken our mentality hard.
"And we're not seeing the same competitive spirit, same purpose, and if we don't clean that up it's not going to get better. It's the only way to get out of difficult positions is to have character and competitive spirit."
"It was a s***ty night," Durant said. "Excuse my language, it was a bad night. We're p***ed. We enjoy basketball.
"We like to win, though, so of course when we lose the games it's going to be a sombre mood in the locker room. But it will change once we start playing some good basketball.
"We got a lot of basketball to be played ahead of us."
While Nash urged his team to up their effort levels, Simmons suggested the Nets are confronting their problems after a meeting between the players following the defeat.
"It was honest," Simmons said of the meeting. "We had a conversation. Obviously, I'm not going to talk about it, but it was honest.
"That's what winning teams do. Hold each other accountable, be able to be open and talk to your team-mates, and respect that and be men."