UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a fight where the approaches failed to mesh, with excessive holding spoiling the contest.
The officials judged it 96-94 for to the winner, with the third a 95-95.
"It wasn't my finest display but what defines champions? They succeed," the victor said in the arena as he was faced disapproval from the spectators.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers thought the challenger, 31, merited the decision but the Derbyshire fighter endured the second defeat of his 28-bout professional journey.
The Champion Secures Triumph in Dull Match
Buatsi's progress has plateaued in the past few years through promotional disputes and long spells of inactivity.
Once tipped as the rising talent of the British fight scene, he now is positioned trying to realize his potential - maybe in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter willing to overcommit in a calculated early stages.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second bout at light-heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who worked behind a protective stance to edge the cleaner periods.
The crowd's patience diminished amid regular grabbing, with Parker going down multiple occasions during close exchanges.
The underdog finished the stronger, but several stanzas were hard to judge. However, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the caliber of power puncher Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Hatton Homage Motivates Manchester Derby Fight
In the supporting bout, Arthur beat Rea by divided decision in a terrific local rivalry battle to win the continental belt.
The venue became quiet for a extremely emotional tribute ahead of the contest as ten bells rang in memory of local hero Ricky Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the final chime ended, the stillness gave way to a roar of "there's just one Hatton".
The fighting during the bout equaled the sentiment. The eventual winner was sent to the canvas by a significant blow flush on the face in the early round, rising shakily to his position on the referee's count.
But the Manchester United-red attire of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was marked up early, significant bruising forming under his face as the fighters landed hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action hardly diminished over the full distance.
Arthur barely secured the result with scoring tallies of draw, close margin and 115-112, but each competitor were relentless and courageous, everything their inspiration Hatton represented.
