UK light-heavyweight Buatsi secured a victory yet disappointing beginning to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a competitive ten-round bout at the boxing arena.
But Parker found success, delivering effective clean blows in a contest where the approaches never quite clicked, with frequent clinching spoiling the fight.
The scoring judges judged it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my greatest display but what defines champions? They succeed," Buatsi commented in the ringside as he was met by negative reactions from the spectators.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Several boxing experts thought the challenger, 31, should have been awarded the decision but the contender endured the second loss of his 28-match professional journey.
The Champion Claims Victory in Uneventful Affair
Buatsi's progress has slowed in the past few years through contract problems and considerable time of lack of fights.
Once tipped as the next star of UK boxing, he now stands trying to fulfil that early promise - perhaps in what feels like a last chance - under new promotional backing.
He began cautiously, with neither fighter willing to overcommit in a calculated early stages.
Parker, whose single earlier setback came at super middleweight against his former opponent in two years ago after damaging his hand in the contest, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who operated with a solid defense to barely take the cleaner rounds.
The spectators' acceptance diminished because of constant clinching, with the challenger going down multiple occasions during close exchanges.
The underdog closed well, but several stanzas were difficult to score. However, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
Buatsi moves on, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Hatton Memorial Fuels All-Manchester Fight
In the secondary fight, Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to win the continental belt.
The arena grew hushed for a profoundly moving tribute before the fight as 10 bells tolled in memory of Manchester's own Ricky Hatton, who passed away aged 46 in recently.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the last bell ended, the quiet became a shout of "there's only one Hatton".
The fighting during the bout equaled the emotion. Arthur was sent to the canvas by a significant blow directly to the jaw in the early round, getting up unsteadily to his footing on the count of eight.
But the traditional red shorts of Arthur soon came forward again. Across the ring, the opponent - in competitive blue - was marked up early, noticeable damage appearing under his orbital area as the pair traded hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the intensity hardly diminished over twelve rounds.
Arthur narrowly won the decision with judges scorecards of 114-114, narrow lead and 115-112, but each competitor were relentless and courageous, exactly what their hero Hatton represented.
