The Blazers have now won three in a row.
The rookie and second-year player gave the Portland Trail Blazers a glimpse Friday night of what they hope to be in the future.
Roy scored 24 points and Jack added 23 as Portland beat a Houston team playing without both Tracy McGrady and Yao Ming 85-78.
The Trail Blazers are also short-handed, playing the third game without leading scorer and rebounder Zach Randolph, who is out with a hand injury that required surgery.
"I think these guys, with a couple of key players missing, have stepped up and raised their level of play,'' Portland coach Nate McMillan said.
Houston led 74-73 with 2:32 remaining before Jack scored five quick points to put the Trail Blazers back on top. His 3-pointer came after Roy, who leads all rookies in scoring, cut through the defense and dished off to him in the corner.
"I think me and Brandon are starting to feed off of each other a little bit,'' Jack said. "We're getting familiar with each other in the backcourt.''
So familiar that Jack told Roy to look for him to be open for that shot during a timeout. And he also warned the Houston fans of his intentions.
"I was joking with them when I ran over there,'' Jack said. "I was like, 'Yo I'm about to hit a 3.''
Shane Battier hit a 3-pointer to tie the game at 70 and gave the Rockets their first lead since it was 4-2 on a shot after a free throw by Jack. The teams exchanged baskets before Jack's five points.
The win breaks a six-game losing streak for the Trail Blazers in Houston and is the Rockets' third straight loss.
"I wanted to be more aggressive and more assertive to make plays,'' Roy said. "These last couple of games I've been able to make some shots and my confidence is going up because I know I'm going to get more shots.''
A free throw by Jack stretched Portland's lead to 81-76 with less than a minute to go before a long 2-pointer by Kirk Snyder cut Houston's deficit to 3. Travis Outlaw made a free throw and Battier missed a 3-point attempt before Freddy Jones added a free throw to ensure the win.
Jack added a pair of free throws with 0.2 seconds remaining.
"We made things interesting there in the fourth quarter,'' Battier said. "But they hit some big shots. That kid Roy is a really good player. Give him a lot of credit.''
Rockets coach Jeff Van Gundy also praised the rookie, calling him "very, very good.''
Portland led by nine before the Rockets scored six straight points, capped by a reverse layup by Chuck Hayes to make it 68-65 with about six minutes remaining. Roy, who had stopped earlier rallies by the Rockets, sat in front of the scorers table helplessly waiting to go in as Hayes hit the layup.
Luther Head, who led Houston with 19 points, cut Portland's lead to one before Roy came through again. He crashed into Hayes as he drove into the lane and both players tumbled to the court. Hayes was called for the foul and Roy got up and rubbed his right knee before hitting both shots.
Roy, who topped 20 points for the third straight game, made two quick baskets midway through the third quarter that pushed Portland's lead back to double figures. The Rockets scored five quick points, including a three by Head to cut the lead to 56-50. Roy responded again, this time calmly sinking a three from the top of the key under heavy pressure just before the shot clock expired.
"We didn't play good enough to win,'' Van Gundy said. "We give up layups down the stretch, 3s in the corner which are 3s we really don't want to give up. We've missed big shots.''
The Trail Blazers had a 17-4 lead with about four minutes remaining in the first quarter after the Rockets made just two of their first 15 shots. Houston went on an 8-3 run before halftime to cut Portland's lead to 40-33 at the break.
Notes: Jamaal Magloire had 12 points and 14 rebounds for his fourth double-double of the season. ... McGrady, who missed eight games earlier this season due to back spasms and soreness, is listed as day-to-day. ... Yao, out with what team officials called a sore lower back, could miss Houston's next game Sunday at Sacramento as well. ... Portland rookie LaMarcus Aldridge, who was taken to a local hospital during Saturday's loss to the Clippers complaining of dizziness and an irregular heart beat, will be re-examined on Monday. McMillan said he is doing better and decisions about his future could be made after the exam. ... Houston's Bonzi Wells, who has been inactive for the last 18 games, had four points and five rebounds in almost 24 minutes on Friday.