UK light heavyweight Buatsi earned a win yet underwhelming start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a close ten-round fight at the boxing arena.
But Parker landed significant shots, landing some telling individual punches in a contest where the approaches never quite clicked, with excessive holding spoiling the fight.
The scoring judges judged it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my greatest performance but how do champions respond? They prevail," the victor stated in the ringside as he was met by disapproval from the spectators.
The British fighter got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers felt Parker, 31, merited the win but the British boxer endured the second setback of his 28-bout professional journey.
Buatsi Earns Triumph in Uneventful Encounter
His career development has slowed in recent times through promotional disputes and considerable time of inactivity.
Once tipped as the future of the British fight scene, he now is positioned trying to realize his potential - possibly in what seems like a final roll of the dice - under recent support.
He opened tentatively, with the two boxers ready to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after breaking his hand mid-fight, was participating in his second fight at the higher weight class.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who utilized a tight guard to narrowly win the better rounds.
The crowd's patience decreased because of frequent holding, with the challenger going down several times during clinches.
The underdog closed well, but many rounds were challenging to assess. Although, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Hatton Homage Fuels Local Rivalry Battle
In the co-main event, Arthur overcame Bradley Rea by split-decision in a terrific local rivalry battle to capture the European light-heavyweight belt.
The arena became quiet for a profoundly moving memorial ahead of the contest as ten bells rang in memory of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the first major televised boxing event in the urban center since Hatton's passing. As the concluding sound ended, the quiet gave way to a roar of "there's only one Ricky Hatton".
The action during the bout equaled the sentiment. Arthur was floored by a significant blow squarely on the chin in the second round, rising shakily to his position on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. From the other corner, Rea - in rival City blue - was marked up soon, noticeable damage forming under his eye as the duo traded hard punches.
The challenger, 27, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the pace hardly diminished over the full distance.
Arthur barely secured the decision with judges scorecards of even score, close margin and 115-112, but each competitor were relentless and full of heart, exactly what their idol Hatton represented.
