UK light-heavyweight Joshua Buatsi earned a win yet disappointing debut to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a close 10-round fight at the boxing arena.
But Parker found success, landing some telling clean blows in a contest where the styles never quite clicked, with excessive holding spoiling the fight.
Two judges scored it 96-94 for to the winner, with the third a 95-95.
"It wasn't my best showing but what do champions do? They succeed," the victor stated in the ringside as he was faced disapproval from the crowd.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in February - to earn a twentieth professional win.
Several boxing experts thought Parker, 31, merited the win but the British boxer suffered the second setback of his 28-match professional journey.
Buatsi Secures Win in Uneventful Encounter
Buatsi's progress has slowed in the past few years through contract problems and long spells of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - maybe in what feels like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers prepared to take risks in a tentative initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after breaking his hand in the contest, was making just his second bout at light heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for the favorite, who worked behind a protective stance to narrowly win the cleaner rounds.
The crowd's patience diminished amid constant clinching, with the challenger slipping to the canvas multiple occasions during grappling.
Parker ended stronger, but several stanzas were challenging to assess. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Hatton Memorial Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a outstanding Manchester vs Manchester contest to win the European light-heavyweight belt.
The arena fell silent for a deeply poignant homage prior to the bout as ten bells rang in honor of Manchester's own Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the final chime disappeared, the silence became a chant of "there's only one Hatton".
The action within the ring equaled the emotion. The eventual winner was sent to the canvas by a left hook directly to the chin in the early round, getting up unsteadily to his position on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged soon, significant bruising developing under his orbital area as the fighters traded hard punches.
The challenger, 27, landed power punches but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the action rarely dropped over twelve rounds.
Arthur narrowly won the result with judges scorecards of even score, close margin and clearer margin, but each competitor were relentless and full of heart, everything their inspiration the legend represented.
