British light heavyweight Buatsi earned a win yet underwhelming start to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a close 10-round bout at the Co-op Live Arena.
But Parker found success, delivering effective individual punches in a contest where the styles never quite clicked, with frequent clinching disrupting the contest.
Two judges judged it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest showing but what defines champions? They win," the victor commented in the ring as he was faced negative reactions from the audience.
The British fighter got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to earn a 20th career victory.
Various analysts felt the challenger, thirty-one, merited the decision but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
The Champion Secures Triumph in Dull Encounter
Buatsi's progress has plateaued in the past few years through promotional disputes and considerable time of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - possibly in what feels like a last chance - under recent support.
He started carefully, with neither fighter prepared to take risks in a cagey opening few rounds.
The challenger, whose sole prior loss came at super middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a tight guard to edge the more effective stanzas.
The spectators' acceptance diminished amid constant clinching, with Parker slipping to the canvas several times during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
Buatsi moves on, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Hatton Homage Inspires Local Rivalry Battle
In the secondary fight, Arthur overcame Rea by close verdict in a terrific all-Manchester fight to win the European light-heavyweight title.
The arena became quiet for a profoundly moving tribute before the fight as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the first major televised boxing event in the urban center since Hatton's passing. As the final chime ended, the stillness gave way to a roar of "there's only one Ricky Hatton".
The fighting during the bout matched the emotion. The eventual winner was floored by a significant blow flush on the jaw in the second round, standing uncertainly to his feet on the count of eight.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was damaged soon, heavy swelling developing under his eye as the duo landed powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with effective uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the result with official scores of draw, close margin and 115-112, but the two athletes were relentless and full of heart, everything their inspiration Hatton represented.
