UK light heavyweight Buatsi made a winning yet disappointing beginning to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a tight ten-round contest at the Manchester venue.
But the challenger found success, landing some telling individual punches in a fight where the styles failed to mesh, with frequent clinching disrupting the action.
The officials scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best showing but what do champions do? They win," the victor said in the ring as he was greeted with boos from the crowd.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Some ringside observers thought the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter suffered the second defeat of his 28-match career.
Buatsi Earns Triumph in Slow Affair
Buatsi's progress has slowed in the past few years through promotional disputes and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to realize his potential - maybe in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors prepared to be aggressive in a cagey opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was participating in his second bout at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who operated with a protective stance to narrowly win the cleaner rounds.
The crowd's patience diminished because of frequent holding, with the challenger slipping to the canvas various instances during clinches.
The underdog closed well, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Hatton Memorial Motivates All-Manchester Contest
In the secondary fight, Arthur beat Rea by divided decision in a terrific local rivalry fight to capture the European light-heavyweight belt.
The arena fell silent for a profoundly moving memorial before the fight as ten bells rang in remembrance of Manchester's own Hatton, who passed away aged 46 in September.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the last bell disappeared, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The fighting within the ring matched the sentiment. The eventual winner was floored by a left hook squarely on the chin in the second round, rising shakily to his feet on the referee's count.
But the Manchester United-red shorts of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was injured early, significant bruising developing under his eye as the fighters exchanged punishing blows.
Rea, 27, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor barely secured the result with scoring tallies of 114-114, 115-113 and wider gap, but the two athletes were relentless and brave, everything their idol Hatton represented.
