British light-heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a tight ten-round bout at the Manchester venue.
But the challenger landed significant shots, connecting with powerful single shots in a contest where the techniques failed to mesh, with frequent clinching spoiling the contest.
Two judges judged it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best display but what do champions do? They win," the victor commented in the ring as he was greeted with boos from the spectators.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to secure a 20th pro success.
Various analysts felt the challenger, 31, should have been awarded the victory but the contender suffered the second defeat of his 28-match professional journey.
The Champion Earns Win in Slow Affair
The fighter's advancement has stalled in recent times through promotional disputes and considerable time of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers prepared to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who utilized a protective stance to narrowly win the better periods.
The audience's tolerance wore thin amid regular grabbing, with Parker going down various instances during clinches.
The underdog ended stronger, but many rounds were challenging to assess. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Hatton Memorial Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Rea by divided decision in a terrific local rivalry contest to secure the European light-heavyweight title.
The arena fell silent for a deeply poignant memorial before the fight as 10 bells tolled in memory of Manchester's own Hatton, who succumbed aged 46 in recently.
It was the premier televised match card in the location since Hatton's death. As the last bell faded, the stillness became a chant of "there's just one Hatton".
The action within the ring equaled the feeling. Arthur was knocked down by a significant blow squarely on the chin in the second round, standing uncertainly to his position on the count of eight.
But the red-colored attire of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was damaged soon, significant bruising appearing under his eye as the pair traded powerful shots.
Rea, 27, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the intensity seldom decreased over 12 rounds.
Arthur barely secured the result with judges scorecards of even score, close margin and 115-112, but each competitor were persistent and courageous, all the qualities their inspiration the legend stood for.
