The teams will meet again in Chicago on Monday.
DeRozan scored a season-best 46 points in a 123-119 loss to the Boston Celtics on Friday.
"He was just spectacular," Bulls coach Billy Donovan said of DeRozan. "He has a refuse-to-die attitude. His scoring kept us close and gave us a chance late. He was incredible."
"(Siakam) slipped there," Raptors coach Nick Nurse said. "Looks like it's something in the groin. We'll get the imaging and stuff done."
The Raptors announced Sunday that they will be without Siakam for at least two weeks. Siakam was diagnosed with a strain of the right adductor muscle.
He has team-high averages in points (24.8), rebounds (9.3) and assists (7.7) in nine games this season.
Toronto already was playing without Fred VanVleet (back) for the third consecutive game.
Nurse said VanVleet could return soon, however.
"We had practice (Thursday), he went through full practice," Nurse said. "I thought he'd probably be going (Friday). I'm expecting him back pretty soon."
He is averaging 23.8 points in his 10 games against the Raptors. DeRozan played nine seasons for Toronto.
"If we're having a great game from DeMar, we're going to play off him," Zach LaVine said. "(DeRozan) was rolling."
It was not enough, however.
"Self-inflicted mistakes," DeRozan said. "The game came down to that."
The Bulls won three of four meetings with the Raptors last season, with the loss coming in Toronto. In the four games, Bulls center Nikola Vucevic had 54 rebounds, a 13.5 per game average.
The Raptors had some notable accomplishments on Friday.
Scottie Barnes recorded his first career triple-double with 11 points, 11 rebounds and 10 assists.
OG Anunoby, who led the Raptors with 27 points, also had five steals. He has 16 steals over the past three games.
"For me, he just looks like he's got his rhythm back," Nurse said of Anunoby. "Maybe getting in better game shape. To me it's noticeable how quick he is to the ball, in gaps, swiping at the ball. It's noticeable here in the last few games.
"That's been his thing since he got into the league, but it just looks like he's moving better and it's showing up at the other end, too. He's getting to the rim better. Finishing better. A lot more composed and balanced at the rim too."
Chris Boucher had 17 points and nine rebounds for the Raptors on Friday and continues to bring energy when he comes in off the bench. Boucher had 17 points and eight boards in the 143-100 win over the San Antonio Spurs on Wednesday.
"Chris was awesome," Nurse said after the game in San Antonio. "He was everywhere. Sometimes he doesn't always do the right thing but he makes up for it by continuing to play really hard. He can cover his own mistakes with his effort and his length. He was everywhere."
Bulls reserve center Andre Drummond (sprained right shoulder) missed his fourth straight game Friday.
--Field Level Media