Horford had 22 points and 10 rebounds and played tough defense with two blocks near the basket in the closing seconds of the Hawks' 93-92 victory Wednesday night.
"It was a do-or-die situation," said Hawks coach Larry Drew, whose team braced for Toronto's final possession during a timeout with 12.6 seconds remaining.
"I told them during the timeout just believe in your defense, go out and get that one stop, and they did. There were bodies flying all over the place."
The game followed a late-afternoon trade which could reshape Toronto's season.
Longtime Raptors guard Jose Calderon and forward Ed Davis gave farewell hugs to their teammates about an hour before the game as word leaked of a trade that will send small forward Rudy Gay from Memphis to Toronto.
With Toronto leading 92-91, a missed jumper by John Lucas set up a Hawks timeout with 27.8 seconds remaining. A few passes around the perimeter set up Horford's open jam to Atlanta the lead.
Horford blocked Kyle Lowry's shot out of bounds with 12.6 seconds remaining. The officials' review of the video confirmed possession belonged to Toronto.
Following a timeout, Alan Anderson missed a long jumper, DeRozan grabbed the rebound and had his attempt under the basket blocked by Horford. There was apparent contact from Horford on DeRozan, but no foul was called.
Coach Dwane Casey and the Raptors wanted a foul called during the intense exchange, but no whistle meant the Hawks survived for the win.
Casey charged onto the court with his arms extended, demanding an explanation for the no-call in the final seconds. He was still upset in his postgame news conference.
"I'm tired of this," Casey said. "I'm tired of losing games because of missed calls at the end of the game. I know the league's going to come down on me for this, but I don't care. Guys have fought their hearts out, played their hearts out and at the end of the game we get cracked, apology, go back to Canada."
Casey said his initial belief a foul should have been called was supported by his review of the video.
"I've been in this league 18 years and I've never seen as many missed calls at the end of the game to cost us the game," Casey said. "We have good officials. Too good to miss calls and short-change these young men like that. And it's not right."
The Hawks had 10 3-pointers, giving them a franchise-record five straight games with at least 10 3s.
Smith said the Hawks "played some pretty good defense" in the closing seconds, though he added with a smile "I don't know what happened" after Anderson's miss.
The Raptors were left with nine players in uniform following the trade.
Calderon and Davis were starters, but Gay, averaging 17.2 points and 5.9 rebounds, could boost the Raptors' chances of moving into the Eastern Conference playoff race.
Toronto entered Wednesday's games in 11th place in the conference, 5 1/2 games behind eighth-place Boston.
Drew said before the game he warned his players to not be distracted by the Toronto trade.
Despite Drew's warning, the Hawks appeared to be sluggish in the first half as they fell behind by 13 points at 53-40 following a 10-0 run by the more aggressive Raptors. Toronto outrebounded the Hawks 28-13 in the half and led 55-45 at halftime.
The Hawks finally answered Drew's call in the third quarter, when they took the lead by outscoring the Raptors 30-14.
Brothers B.J. and Justin Upton, added to the Braves' outfielder through free agency and a trade, respectively, had front-row seats, one day after Justin had his introductory news conference in Atlanta. ... Gray had 10 rebounds in the first half, matching his season high for a game. He had 22 points and 10 rebounds in Toronto's 112-102 loss to Golden State on Monday night. ... The Raptors finished with a 52-36 advantage in rebounds.