Allen got hurt in the first quarter, when he also suffered a bruised quadriceps while driving to the basket. He went to the locker room, but the Cavs held on to win 104-96 and tighten their grip on the No. 6 spot in the Eastern Conference.
The team did not provide a timetable for Allen's return. It's safe to assume he'll miss at least a few games, depending on the severity of the fracture to his non-shooting hand. Cleveland begins a three-game trip at Indiana on Tuesday.
The 7-foot player has had a major role in Cleveland's rise this season, and now the Cavs could be without him for a key stretch of their playoff push.
"He's the anchor of our defense," forward Kevin Love said after Monday's practice. "He means so much to this team."
Allen is averaging 16.1 points, 10.8 rebounds and 1.3 blocks in 56 games for the Cavs, who are trying to make the postseason for the first time since 2018 after winning just 22 games last season.
His injury is just the latest for Cleveland, which has been dealing with medical issues all season. Beginning with leading scorer Collin Sexton suffering a season-ending knee injury in November, the Cavs have had to juggle lineups and persevere.
They're still missing guard Caris LeVert (sprained foot), who has played just four games since coming over in a trade last month, and backup point guard Rajon Rondo. He was signed after Ricky Rubio tore a knee ligament.
To this point, the Cavs have managed to stay competitive. Their depth will face another tough test.
"It's definitely been tough," said Love, who could be moved back into the starting lineup after coming off the bench. "Everybody has been wrecked by COVID, but even take that out, we've had guys miss a lot of games. and I'm just glad we have so many damn pros on this team that have stepped up for us."
After Allen went down, Cavs rookie Evan Mobley stepped up with 20 points and 17 rebounds against the Raptors. The 20-year-old has been unflappable in his first season, and the Cavs will need to count on him more than ever.
"We're going to need him to have more nights like he had last night," Love said. "And while that's a lot of responsibility, I think that he's more than capable and more than ready for it. He's learning on the fly."