British light heavyweight Buatsi earned a win yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a competitive 10-round fight at the boxing arena.
But Parker landed significant shots, landing some telling single shots in a match where the techniques failed to mesh, with excessive holding disrupting the contest.
Two judges scored it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my greatest showing but how do champions respond? They win," Buatsi stated in the ring as he was greeted with boos from the crowd.
The Londoner bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to claim a 20th career victory.
Various analysts felt the challenger, thirty-one, should have been awarded the win but the Derbyshire fighter experienced the second loss of his 28-bout professional journey.
The Champion Secures Triumph in Slow Affair
Buatsi's progress has slowed in recent times through contract problems and long spells of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers prepared to overcommit in a calculated opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second fight at light-heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for the favorite, who utilized a solid defense to edge the better stanzas.
The crowd's patience wore thin during constant clinching, with the challenger falling to the floor various instances during grappling.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the likes of power puncher Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Hatton Tribute Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a outstanding Manchester vs Manchester battle to secure the European light-heavyweight championship.
The stadium grew hushed for a profoundly moving memorial prior to the bout as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who passed away aged 46 in recently.
It was the premier televised fight night in the city since Hatton's death. As the concluding sound faded, the stillness transitioned to a chant of "only one Hatton".
The fighting during the bout equaled the emotion. The eventual winner was sent to the canvas by a left hook squarely on the face in the initial stage, standing uncertainly to his feet on the count of eight.
But the traditional red shorts of the fighter soon advanced again. From the other corner, the opponent - in competitive blue - was damaged soon, heavy swelling developing under his eye as the fighters traded hard punches.
Rea, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the intensity hardly diminished over twelve rounds.
Arthur edged the verdict with judges scorecards of 114-114, narrow lead and clearer margin, but each competitor were determined and full of heart, everything their inspiration Hatton stood for.
