British light heavyweight Buatsi earned a win yet underwhelming beginning to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But Parker found success, delivering effective single shots in a contest where the techniques never quite clicked, with frequent clinching marring the contest.
The scoring judges had it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They prevail," Buatsi stated in the ring as he was faced boos from the spectators.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers thought Parker, thirty-one, deserved the decision but the British boxer endured the second setback of his 28-bout career.
The Champion Earns Victory in Uneventful Match
Buatsi's progress has stalled in the past few years through management issues and extended periods of lack of fights.
Previously predicted as the future of British boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under fresh management.
He started carefully, with both competitors ready to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after damaging his hand in the contest, was making just his second fight at the higher weight class.
By the middle rounds, Parker had kept it close, making life awkward for Buatsi, who operated with a tight guard to narrowly win the better periods.
The spectators' acceptance diminished amid regular grabbing, with the challenger slipping to the canvas various instances during clinches.
The underdog ended stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the caliber of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a outstanding local rivalry battle to capture the European light heavyweight title.
The venue grew hushed for a extremely emotional tribute before the fight as 10 bells tolled in memory of the city's native Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the last bell ended, the quiet became a chant of "there's only one Ricky Hatton".
The fighting during the bout matched the feeling. The eventual winner was knocked down by a left hook directly to the chin in the early round, getting up unsteadily to his feet on the count of eight.
But the traditional red shorts of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was marked up early, noticeable damage appearing under his orbital area as the duo exchanged hard punches.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over twelve rounds.
The victor edged the verdict with scoring tallies of 114-114, 115-113 and wider gap, but each competitor were persistent and courageous, exactly what their inspiration the legend represented.
