UK light heavyweight Buatsi made a winning yet underwhelming beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a competitive 10-round bout at the Manchester venue.
But Parker landed significant shots, landing some telling clean blows in a match where the styles didn't complement each other, with excessive holding marring the contest.
Two judges scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my finest showing but what do champions do? They prevail," Buatsi commented in the ringside as he was faced boos from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in February - to secure a 20th career victory.
Some ringside observers believed the challenger, 31, should have been awarded the victory but the British boxer experienced the second loss of his 28-match professional journey.
The Champion Claims Triumph in Uneventful Match
Buatsi's progress has plateaued in recent times through management issues and considerable time of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with the two boxers prepared to overcommit in a tentative opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after breaking his hand in the contest, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who utilized a solid defense to barely take the cleaner rounds.
The audience's tolerance wore thin during constant clinching, with the challenger slipping to the canvas various instances during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. However, previous titleholder the boxing analyst - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
Buatsi advances, but with the caliber of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Homage Motivates Local Rivalry Contest
In the co-main event, Arthur beat Rea by split-decision in a terrific all-Manchester fight to win the European light heavyweight belt.
The arena became quiet for a deeply poignant memorial ahead of the contest as 10 bells rang in memory of Manchester's own Ricky Hatton, who died at age 46 in last month.
It was the first major televised match card in the location since Hatton's passing. As the last bell ended, the quiet became a chant of "there's only one Hatton".
The competition within the ring reflected the sentiment. Arthur was knocked down by a powerful punch squarely on the chin in the second round, rising shakily to his position on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up early, significant bruising forming under his orbital area as the pair exchanged powerful shots.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur narrowly won the verdict with judges scorecards of even score, close margin and clearer margin, but the two athletes were determined and courageous, everything their idol the legend embodied.
