British light heavyweight Buatsi earned a win yet disappointing debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a close ten-round bout at the boxing arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the styles failed to mesh, with excessive holding disrupting the action.
The officials had it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best showing but how do champions respond? They prevail," the victor said in the arena as he was met by boos from the spectators.
The British fighter got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to earn a 20th career victory.
Various analysts felt the challenger, 31, merited the win but the British boxer endured the second setback of his 28-fight career.
The Champion Earns Win in Uneventful Match
The fighter's advancement has slowed in recent times through management issues and long spells of lack of fights.
Previously predicted as the future of the British fight scene, he now stands trying to realize his potential - maybe in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with both competitors willing to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was competing in only his second appearance at the higher weight class.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who operated with a tight guard to narrowly win the better stanzas.
The spectators' acceptance diminished during frequent holding, with the challenger slipping to the canvas several times during clinches.
Parker ended stronger, but multiple periods were challenging to assess. Although, former world champion the boxing analyst - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just a single round.
The victor moves on, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Hatton Homage Fuels All-Manchester Battle
In the co-main event, Arthur beat Bradley Rea by divided decision in a outstanding Manchester vs Manchester battle to win the European light heavyweight championship.
The arena grew hushed for a deeply poignant tribute ahead of the contest as 10 bells sounded in memory of the city's native Hatton, who died at age 46 in recently.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the final chime disappeared, the quiet became a chant of "there's only one Hatton".
The fighting during the bout equaled the sentiment. Arthur was sent to the canvas by a left hook squarely on the chin in the early round, rising shakily to his position on the eight count.
But the traditional red attire of the fighter soon advanced again. Across the ring, the opponent - in competitive blue - was marked up early, heavy swelling appearing under his face as the pair exchanged hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity seldom decreased over twelve rounds.
Arthur barely secured the result with scoring tallies of even score, narrow lead and 115-112, but each competitor were persistent and brave, all the qualities their idol the legend stood for.
