British light-heavyweight Buatsi made a winning yet disappointing start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful individual punches in a fight where the styles never quite clicked, with frequent clinching spoiling the contest.
The officials judged it 96-94 for to the winner, with the final judge a even score.
"It wasn't my greatest display but what defines champions? They prevail," Buatsi said in the ring as he was greeted with disapproval from the crowd.
The Londoner bounced back after his initial pro loss - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Several boxing experts thought the challenger, 31, merited the win but the contender experienced the second setback of his 28-fight career.
The Champion Secures Triumph in Dull Affair
His career development has plateaued in recent years through management issues and considerable time of lack of fights.
Once tipped as the future of the British fight scene, he now is positioned trying to achieve expectations - maybe in what seems like a last chance - under recent support.
He began cautiously, with neither fighter willing to take risks in a calculated opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in two years ago after breaking his hand during the bout, was making just his second appearance at the higher weight class.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who utilized a protective stance to edge the cleaner periods.
The spectators' acceptance diminished during frequent holding, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but several stanzas were hard to judge. Although, former world champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
The victor advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Hatton Tribute Motivates Local Rivalry Battle
In the co-main event, Arthur beat Bradley Rea by divided decision in a terrific local rivalry battle to secure the continental belt.
The arena became quiet for a profoundly moving tribute before the fight as a bell ceremony sounded in memory of Manchester's own Hatton, who succumbed at age 46 in last month.
It was the premier televised fight night in the location since the passing of Hatton. As the final chime faded, the stillness became a shout of "only one Hatton".
The competition inside the ropes matched the emotion. Arthur was sent to the canvas by a significant blow flush on the face in the second round, getting up unsteadily to his position on the count of eight.
But the traditional red shorts of Arthur soon came forward again. Across the ring, the opponent - in rival City blue - was damaged soon, significant bruising forming under his face as the fighters exchanged powerful shots.
Rea, 27, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the intensity hardly diminished over the full distance.
The victor narrowly won the verdict with official scores of 114-114, narrow lead and clearer margin, but the two athletes were persistent and brave, everything their inspiration the legend embodied.
