Nelson, the team's returning leading scorer and rebounder from last season and an all Pac-10 selection, will not be allowed to practice or play with the team until further notice, Howland said.
This suspension was handed down after Nelson was seen laughing and joking with teammates and fans in the waning moments of UCLA's 69-59 loss to Texas on Saturday after he was benched in the second half because of poor play in the first half.
"This continues a trend of very disappointing behavior by Reeves," Howland said. "I personally have worked hard with him to illustrate the importance of the code of conduct for our student-athletes, as well as the ramifications for violating it. I am disappointed that he has continued to fall short of my expectations."
Forward David Wear said, "In general, I don't think that's something that you want to do. You don't want to be laughing when your team is losing."
Nelson was suspended Nov. 14 after failing to participate in team huddles during timeouts of the Bruins' season opener against Loyola Marymount and then getting into a verbal altercation with Howland the next day at a team film session.
He sat out one game and was reinstated for UCLA's three-game tournament in Maui, but missed the team plane to the tournament and found his own way there. He has not started since and has averaged only 4.2 points and 3.8 rebounds in 16.8 minutes over the last five games.
"It's been very difficult," Howland said of dealing with Nelson's mercurial personality. "It's definitely a distraction and we don't need any distractions. It's tough enough to come together without being distracted as a group."
Howland wouldn't commit to a length of the current suspension, saying only that he wouldn't practice this week or play Saturday against Pennsylvania.
"I want him to have some time away," Howland said. "It is a privilege and an honor to be a part of the UCLA basketball program and it's not something to be taken for granted."
Howland has not completely ruled out the possibility of permanently dismissing Nelson from the team. He has had one brief conversation with Nelson and plans to meet with the 6-foot-8 power forward again after final exams are over later this week.
Howland has also kept Nelson off-limits to reporters recently.
Nelson has a history of volatile moments on the court with outbursts directed at teammates and periods of going into a shell dotting his career. He's also had moments of brilliance and averaged 13.9 points and 9.1 rebounds last season.
Howland hinted that Nelson's troubles are more far-reaching than just a bad attitude and, without going into further detail, said he felt obligated to try and help Nelson work through his issues.
"These kids aren't professional athletes, they're college kids," Howland said. "I work very hard to try to work with each kid but as it reflects and affects the team as a whole, there is a point where enough is enough. Are we there at this point yet? We'll see with our next meeting."
Howland said Nelson has been very apologetic and reserved in meetings regarding his behavior. He called Nelson "contrite" and "reflective" and that makes it more difficult to give up on him.
But Howland also said that Nelson can't continue to be a distraction to a team and a program with a proud tradition.
"As talented as a player as Reeves is, he's got to understand that no one is bigger than the team and the team comes first," Howland said. "And there's certain adherence of principles and values of being a UCLA basketball player that need to be followed."
Nelson's teammates found out shortly before practice Tuesday that he was in trouble again. It's finals week on campus, so the players have been juggling practice with test taking.
"We all know how good of a player Reeves is. He's a good friend. Of course we want him on the team," starting guard Lazeric Jones said.
Jones said he doesn't understand exactly what's going on with Nelson.
"It's at a higher level. It's with coach," he said. "If it's with coach, I don't really want to step in between there."
Peter Yoon covers UCLA for ESPNLosAngeles.com. Information from The Associated Press was used in this report.