UK light heavyweight Buatsi earned a win yet underwhelming debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a majority decision in a close ten-round contest at the Manchester venue.
But Parker landed significant shots, connecting with powerful clean blows in a match where the approaches didn't complement each other, with constant grabbing spoiling the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my greatest display but how do champions respond? They prevail," Buatsi said in the arena as he was faced disapproval from the audience.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in February - to earn a 20th career victory.
Various analysts believed Parker, thirty-one, deserved the win but the British boxer suffered the second setback of his 28-bout professional journey.
Buatsi Claims Win in Dull Encounter
Buatsi's progress has slowed in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to realize his potential - maybe in what appears to be a last chance - under new promotional backing.
He began cautiously, with the two boxers ready to overcommit in a tentative initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after damaging his hand in the contest, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a protective stance to edge the cleaner stanzas.
The audience's tolerance diminished because of frequent holding, with Parker slipping to the canvas various instances during clinches.
The underdog ended stronger, but several stanzas were hard to judge. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
The victor moves on, but with the likes of power puncher Benavidez, and Russian champion Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires Manchester Derby Battle
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a outstanding all-Manchester fight to win the European light heavyweight belt.
The venue became quiet for a deeply poignant memorial prior to the bout as ten bells rang in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the last bell faded, the stillness gave way to a shout of "only one Hatton".
The fighting during the bout matched the sentiment. The eventual winner was sent to the canvas by a powerful punch flush on the face in the second round, standing uncertainly to his feet on the referee's count.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was marked up early, significant bruising developing under his face as the duo landed hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur edged the decision with scoring tallies of 114-114, close margin and clearer margin, but both fighters were relentless and courageous, exactly what their inspiration Hatton stood for.
