Roy scored 23 points, Aldridge added 19 points and 12 rebounds and the Trail Blazers rallied in the fourth quarter for an 85-81 victory Tuesday night.
The win lost some of its luster when starting center Joel Przybilla became the most recent Portland player to get hurt, injuring his right knee in the first quarter.
Przybilla went up for a rebound and came down awkwardly on his right leg with 3:12 left, falling to the ground in pain. He ruptured his right patella tendon and dislocated his patella.
The Trail Blazers, who began the game with 11 healthy players, announced in the third quarter that Przybilla would be out indefinitely.
"We keep losing guys and it's nerve-racking just to take the floor," coach Nate McMillan said. "This was a great win for our group under the conditions."
Portland has already lost center Greg Oden for the season with a left knee injury.
"I couldn't believe that somebody else got hurt," said Aldridge, a former Dallas-area high school star.
Portland had come up empty in its last 11 trips to Dallas before the Trail Blazers earned their first road victory over the Mavericks since Jan. 26, 2003.
Nowitzki had 27 points and nine rebounds after missing the previous game and a half with a deep cut on his right elbow, and J.J. Barea added 22 points for the Mavericks, who had won six of seven.
The Mavericks played poorly in the first and fourth quarters, and they missed 10 of 22 free throws.
"The free throw line was one of the negative things," coach Rick Carlisle said. "We were never quite right all night and Portland took advantage of it."
Dallas held a 70-64 lead early in the fourth but Portland came back with a 17-3 run that began and ended with 3-pointers from Steve Blake for an 81-73 lead with 3:48 left.
Nowitzki's 3-pointer with 1:16 left pulled Dallas to 83-81. The Mavericks had a chance to tie it, but Nowitzki's 17-footer rimmed out with 40 seconds to play.
Andre Miller's two free throws with 6.5 seconds remaining sealed the victory.
Nowitzki tested his heavily bandaged and padded elbow during a pregame shooting session at the team's practice gym and felt OK to play.
"I felt pretty good," he said. "I wasn't worried about getting the ball up and shooting, but I was worried about hitting it while I was boxing out."
Nowitzki was injured Friday night in a collision with Houston's Carl Landry during an overtime loss to the Rockets. Nowitzki had two of Landry's teeth embedded in his elbow and needed extensive treatment to clean the wound.
Nowitzki missed the second half against Houston and sat out Sunday night's 102-95 victory over the Cleveland Cavaliers. His teammates welcomed him back.
"I'm not surprised," Barea said. "He made sure he was ready to play."
Dallas labored through a 33-point first half, its lowest-scoring first half of the season, before heating up in the third quarter.
Nowitzki and Barea each scored 11 points in the third to help the Mavericks build a 65-58 advantage heading into the final quarter.
Except for the third quarter, Dallas' offense was out of sync all night.
"Other than the third quarter, we get beat by 20 points," Carlisle said.
Portland held a 44-33 halftime lead. Nowitzki played the last 6 minutes of the second quarter, and the Blazers went on a 17-4 run during that stretch.
Roy injured his left shoulder late in the fourth quarter, but said he would probably be able to play Wednesday night at San Antonio. ... McMillan will remain on crutches for the next three weeks while recovering from a ruptured right Achilles' tendon. The team has permission from the NBA for Blazers assistants to take over some of the in-game coaching duties such as calling timeouts while McMillan is hobbled. ... Dallas shot 26 percent (6 for 23) and had 14 points in the opening quarter, its lowest-scoring first quarter of the season. ... Portland's Juwan Howard had 10 points and 10 rebounds, his first double-double since April 6, 2007, for Houston.