UK light-heavyweight Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a close ten-round contest at the boxing arena.
But the challenger had his moments, delivering effective single shots in a contest where the approaches never quite clicked, with constant grabbing marring the action.
The scoring judges scored it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my finest showing but what do champions do? They succeed," the victor said in the ringside as he was faced disapproval from the crowd.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to claim a 20th career victory.
Several boxing experts believed Parker, thirty-one, deserved the decision but the Derbyshire fighter endured the second loss of his 28-match professional journey.
The Champion Secures Victory in Uneventful Affair
Buatsi's progress has plateaued in recent years through management issues and considerable time of inactivity.
Earlier forecast as the rising talent of UK boxing, he now stands trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under recent support.
He started carefully, with the two boxers prepared to overcommit in a calculated early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was making just his second fight at the higher weight class.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who worked behind a solid defense to edge the more effective stanzas.
The spectators' acceptance diminished amid regular grabbing, with the challenger going down multiple occasions during clinches.
The underdog closed well, but several stanzas were challenging to assess. Although, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
The victor advances, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Hatton Tribute Inspires All-Manchester Battle
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a outstanding Manchester vs Manchester fight to capture the continental championship.
The arena fell silent for a deeply poignant memorial ahead of the contest as a bell ceremony rang in remembrance of local hero Hatton, who passed away at age 46 in last month.
It was the first major televised boxing event in the location since the passing of Hatton. As the concluding sound faded, the stillness transitioned to a roar of "there's only one Ricky Hatton".
The competition during the bout matched the feeling. The eventual winner was knocked down by a left hook flush on the face in the initial stage, rising shakily to his position on the count of eight.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, the opponent - in opposing blue - was damaged quickly, heavy swelling appearing under his eye as the fighters landed hard punches.
Rea, 27, delivered effective strikes but the champion, 34, fought fire with fire with stinging uppercuts as the action hardly diminished over twelve rounds.
The victor barely secured the decision with scoring tallies of draw, narrow lead and 115-112, but both fighters were determined and full of heart, all the qualities their hero the legend embodied.
