The Montreal light heavyweight (27-2) used his right hand exclusively from early on in the 10-round bout on Friday night to score a unanimous decision over Aleksy Kuziemski (23-5) at the Bell Centre.
Pascal said he hurt his left shoulder in the second round, and threw only rights from the fourth round on, but still managed to knock his 35-year-old Polish opponent down in the sixth and 10th rounds.
"I didn't give my best performance, but I did the best I could," said Pascal, who had not fought since May, 2011, when he lost the WBC title to veteran Bernard Hopkins. "I only had one hand.
"I'm proud of myself. I hope that people understand."
It got worse for the right hander in the final round, when he also injured his right hand, but he danced away from Kuziemski in the final minute despite booing from some in the crowd of 6,100 who wanted to see a knockout.
"I knew I was ahead on the scorecards, but I'm a crowd-pleaser and I wanted the knockout," he said. "But give him credit. He's a tough guy."
Two ringside judges scored it 98-90 and the other had it 100-88 for the 30-year-old Pascal. The Canadian Press scored it 97-91.
Promoter Yvon Michel said Pascal will undergo an MRI exam on Sunday to determine the extent of his injuries.
The results will determine whether he will be healthy enough to take part in a scheduled title fight on March 23 on the HBO specialty channel against (Bad) Chad Dawson, who took the championship belt from Hopkins in April.
Pascal took time off to get over his loss on points to then-45-year-old Hopkins, then had to pull out of two scheduled bouts due to shoulder and hand injuries. It was the longest layoff of his career.
"It's disappointing, but I don't think I'm a fragile athlete," he said.
Trainer Marc Ramsay said the shoulder made for anxious moments in Pascal's corner because he couldn't follow his game plan with only one arm.
"But we saw he was still winning the rounds," said Ramsay. "He used his legs and tried to surprise him.
"Sometimes he faked with the left. Everyone stayed calm and he was still able to dominate."
Pascal won the WBC title in 2009 and defended it four times, including an 11-round win over Dawson and a draw in his first meeting with Hopkins in December 2010.
He had also fought one-handed, that time with the left, in a title defence against Adrian Diaconu.
In the co-feature, David Lemieux (28-2) scored his third straight victory in under two rounds as the Montreal middleweight backed Russian Albert Ayrapetyan into a corner and knocked him out with a left-right combination early in round two of their scheduled 10-rounder. Ayrapetyan was down twice in the opening round as well.
The free-swinging Lemieux is scheduled to fight for the minor NABF and NABO titles on Feb. 8 at the Bell Centre against Jose Miguel Torres (26-5) of Colombia.
Michel called it "the best performance I've ever seen from David Lemieux," because the 23-year-old showed improved technique and patience without giving up any power in his punches.
It was only the second time Ayrapetyan was stopped before the limit in a fight since he lost in 2005 to Sergio Martinez, the current WBC middleweight champion.
On the undercard, Eleider Alvarez (11-0), a Colombia native considered a promising light heavyweight prospect, dominated every round but it went to the eighth before he knocked Englishman Danny (Big Mac) McIntosh (13-4) down and out to retain the minor NABO title.
Super featherweight Logan McGuinness (19-0-1), the minor NABA champion from Orangeville, Ont., lost his first four rounds but turned his bout around with a knockdown from a body shot in the fifth and went on to a 10-round decision over Carlos Manuel Reyes (23-2-1) of the Dominican Republic.
Kevin Bizier (19-0) was all over Doel Carrasquillo (16-23-1) for eight rounds but the Quebec City middleweight wasn't able to put away the clowning American. He got the unanimous decision.
There were three Hungarians on the undercard and all lost in a total of nine minutes 10 seconds. Featherweight Marc (Gwapo) Pagcaliwangan (2-0) of London, Ont., heavyweight Oscar Rivas (12-0) of Montreal, who scored first round knockouts, and light heavyweight Andrew Gardiner (6-0) of Ottawa were the winners.