I've been watching the Nets since they picked up Green, and they're not as bad as their record appears. If they were healthy and had Green+Wallace all season, they're probably a 45-50 win type team.
Unfortunately they still don't have Lopez, and it looks like Wallace is out for the season now too (along with Jordan Farmar), and what's left is a team that can still score respectably but is an absolute disaster on defense. Avery Johnson has called Green his third best defender behind Deron and Wallace, and while Green has made some huge strides in that area, eh, he shouldn't be any team's best defender on the wings.
If the Celtics can take them out of their rhythm offensively, then they'll be fine. On the other hand, if they run into a hot shooting night from Green like the Sixers did last night, the Nets are entirely capable of beating a team like the Celtics.