Ujiri said he didn't have a problem with Gay's talent level or attitude. But he said that the Raptors were lacking in chemistry, and changes had to come.
So Ujiri pulled off his second major trade since joining the Raptors in the off-season, sending Gay and his sizable contract to the Sacramento Kings.
"Phenomenal talented player, it just wasn't working," Ujiri said at a press conference Monday.
Ujiri said there were issues with spacing on the floor, and also acknowledged the suggestions that Gay and guard DeMar DeRozan did many of the same things on the court.
The seven-player deal also sees centre Aaron Gray and forward Quincy Acy head to the Kings.
The Raptors receive guards John Salmons and Greivis Vasquez, forward Patrick Patterson and centre Chuck Hayes in return.
For his part, Gay was looking forward to a fresh start with the Kings.
"Excited to say the least," Gay told reporters in Sacramento before the Kings hosted the Dallas Mavericks on Monday night. "I have a couple guys with me that came with me. It's a new beginning. I've been saying it all day. They have a lot of young pieces and I'm glad to be a part of it."
Gay averaged 19.4 points per game, 7.4 rebounds and 2.2 assists for the Raptors this season.
The move will free up significant cap space for the Raptors.
The six-foot-eight, 230-pound Gay was the Raptors' highest paid player with a contract that pays him US$17.8 million this season with a $19.3-million player option for next season.
The team also announced Monday that it has waived back-up point guard D.J. Augustin.
A first-round pick of the Houston Rockets in 2006, Gay came to Toronto from Memphis on January 30 in a three-team deal that sent Ed Davis to the Grizzlies and Jose Calderon to Detroit.
Gay posted averages of 19.5 points, 6.8 rebounds and 35.0 minutes in 51 contests (50 starts) with Toronto.
Gay scored 20 or more points 27 times for the Raptors, with four games of 30 or more points. He was ranked second on the Raptors in scoring (19.5) and rebounds (7.4) this season.
While Gay often put up big scoring numbers, it often took him a lot of shots to do it. This season has been particularly troubling as Gay has a 38.8 per cent field goal percentage, well off his career average of 44.7 per cent.
"To be fair on Rudy, he wasn't having the normal shooting years that he's had," Ujiri said. "He was going through a tough stretch I think.
"That wasn't necessarily the case on what made our decision. I think it was more of just chemistry. Everyone could see there wasn't much sync there."
Ujiri has wasted no time putting his stamp on this team. Just 19 games into the 2013-14 season, the team is already looking vastly different to the squad that ended the last campaign.
One of Ujiri's first moves as GM was to trade out-of-favour forward-centre Andrea Bargnani — once considered the future of the franchise — to the New York Knicks. Like Gay, Bargnani was considered difficult to move because his contributions did not match his hefty contract.
Others gone after last season include Alan Anderson, John Lucas III, Linas Kleiza and Sebastian Telfair.
The other players Sacramento gets in the deal are role players off the bench.
Acy, at 6-7, 225-pounds, is averaging 2.7 points and 2.1 rebounds off the bench. The 23-year-old is making just $788,872 with one year left on his contract after being drafted 37th overall by the Raptors in 2012.
Gray, a seven-foot, 270-pound 29-year-old bench player, is averaging 1.3 points and two assists. He's in the last year of a contract that pays him $2.6 million. He scored a career-high 22 points with the Raptors on January 28 versus Golden State.
The player who should make the biggest impact on the Raptors coming the other way is likely Vasquez. The six-foot-six, 211-pound guard is known as a decent ball distributor, which should help a team ranked dead last in the league in assists.
Vasquez has averaged 9.8 points, 1.9 rebounds, 5.3 assists and 25.8 minutes in 18 games with the Kings this season.
Vasquez played with Memphis and New Orleans prior to joining the Kings. In 232 career games (123 starts), he has averaged 9.0 points, 5.6 assists, 2.7 rebounds and 24.6 minutes.
"He's a tough kid, great basketball IQ," Ujiri said. "He'll give us something, and we're happy to bring him in as a piece and see what he can bring to this organization."
The native of Caracas, Venezuela, brings a cap hit of $2.1 million. His contract expires at the end of the season.
Salmons, 33, is a 12-year NBA veteran who has serviceable career averages of 9.8 points, 3.0 rebounds and 2.5 assists, though that drops to 5.8 points, 2.6 rebounds, 2.4 assists so far this season.
He may be best known in Toronto for entering a verbal agreement to play here in 2006, before backing out and joining Sacramento instead.
"I didn't really feel (Toronto) was where God was leading me," Salmons said at the time.
With a cap hit of $7.5 million to Toronto this season with $7 million more in 2014-15, the 6-7, 210-pound Salmons is the most expensive player coming from the Kings
The 30-year-old Hayes, a 6-6, 250 pounder, makes $5.7 million this season and $5.9 million next season. He averages 2.1 points and 2.9 rebounds.
Patterson, a 6-9, 235-pound 24-year-old, makes $3.1 million this season while averaging 6.9 points and 5.8 rebounds.
With files from The Associated Press.