Butler had 35 points and connected on all five 3-pointers, Jamison added 27, and the Wizards handed the Dallas Mavericks their first home loss of the season and third straight overall, 110-98 on Monday night.
Arenas, a three-time All-Star who averaged 28.4 points last season, missed his sixth straight game. Washington has won four times during that span.
"Never underestimate our will," Butler said. "Guys were aggressive and stayed positive. It's not going to be one individual [replacing Arenas]. We've just got to continue to keep this thing going."
Butler has never been known as a 3-point specialist, carrying a career mark of 30 percent beyond the arc into this season.
But he worked on his deep shot over the summer, and the long hours of practice seem to be paying dividends.
"I got a real nice rhythm after getting a day off," Butler said. "I got rested. The coaches told me not to hesitate because once you hesitate, your legs aren't as much under you. That's what I worked on all summer."
Washington coach Eddie Jordan called it "a crucial win for us. It shows what we can do."
Dallas, which started the season 6-0 at home, got 31 points, eight rebounds and six assists from Dirk Nowitzki, but the reigning league MVP didn't get enough help from his teammates.
Jason Terry, who was averaging 18.5 points per game, went 1-for-9 from the field for the Mavericks and finished with season-low three points. And bench sparkplug Jerry Stackhouse went 2-for-9 from the floor and had six points.
"Maybe tonight I should have shot more," Nowitzki said. "But when I see a teammate open, he's going to get the ball. [Terry] isn't going to go 1-for-9 every night. We couldn't make enough defensive plays. Whatever they wanted, they got."
Washington converted 50.7 percent of its field goal attempts (38-for-75) with the help of some lackluster Dallas defense.
"It's always defense," Mavericks coach Avery Johnson said. "If we don't get stops, we can't get in our running game. Once we get to a level of consistency, we'll be pretty good again."
Antonio Daniels had a season-high 16 points for the Wizards, who ended a two-game losing streak and snapped a streak of five straight defeats in Dallas.
Dallas lost its third in a row to an Eastern Conference opponent and fell to 1-4 against the East despite hitting all 29 free throw attempts for a club record, breaking the previous mark of 27 set in February of 2002 against the New Jersey Nets.
Last season the Mavericks went 27-3 against the East.
In their previous two home games, the Mavericks rallied from a 24-point deficit to beat Toronto and from 10 points down to defeat Memphis. But they weren't able to do the same against the Wizards, who got 14 third-quarter points from Butler for an 85-72 edge heading into the final period.
Dallas got a second-half spark from Barea, who scored 11 of his team's 13 points during one stretch to pull within 85-77 early in the fourth quarter.
The Wizards answered with a 6-0 run capped by Brendan Haywood's tip-in with 9:32 left for a 91-77 lead, and the Mavericks were never able to work the deficit lower than nine the rest of the way.
Back-to-back 3-pointers by Butler, the second with 4:45 left in the third quarter, capped a 16-7 spurt that sent the Wizards to a 70-59 edge.
Dallas G Eddie Jones will be out for a few more games because of a sore right leg. Johnson said Jones needs to drop a few pounds before he will be cleared for game action. "We'll see how bad he wants to play," Johnson said. ... Mavericks F Devean George, out for the first 14 games with a left foot injury, will begin full practices this week and is close to joining the rotation. ... Washington hadn't won in Dallas since Dec. 8, 2001. ... Howard had a four-point play midway through the second quarter, hitting a 3-pointer and a free throw.
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