UK light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But Parker found success, delivering effective clean blows in a contest where the approaches failed to mesh, with constant grabbing disrupting the contest.
The officials scored it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my finest showing but what defines champions? They succeed," the victor said in the ringside as he was greeted with disapproval from the spectators.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts believed Parker, 31, should have been awarded the victory but the contender experienced the second setback of his 28-bout professional journey.
Buatsi's progress has slowed in recent years through promotional disputes and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now stands trying to fulfil that early promise - possibly in what seems like a last chance - under recent support.
He started carefully, with the two boxers prepared to overcommit in a tentative initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after suffering a hand injury in the contest, was competing in only his second fight at light heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who worked behind a tight guard to edge the more effective rounds.
The crowd's patience diminished during constant clinching, with Parker slipping to the canvas multiple occasions during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. Although, former world champion Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
Buatsi progresses, but with the caliber of hard hitter Benavidez, and Russian star Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a excellent Manchester vs Manchester contest to win the continental title.
The stadium became quiet for a deeply poignant homage ahead of the contest as ten bells sounded in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast match card in the location since Hatton's death. As the final chime disappeared, the silence gave way to a chant of "there's only one Ricky Hatton".
The action during the bout reflected the emotion. The eventual winner was sent to the canvas by a significant blow squarely on the chin in the initial stage, getting up unsteadily to his position on the eight count.
But the red-colored equipment of Arthur soon came forward again. Across the ring, Rea - in competitive blue - was injured early, heavy swelling forming under his face as the fighters exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the intensity seldom decreased over 12 rounds.
Arthur edged the decision with official scores of 114-114, 115-113 and 115-112, but the two athletes were relentless and full of heart, all the qualities their idol the legend represented.
Elena Hartwell is a multidisciplinary artist and educator passionate about fostering creative communities in London.