UK light-heavyweight Joshua Buatsi secured a victory yet disappointing start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a competitive ten-round bout at the boxing arena.
But Parker found success, delivering effective single shots in a match where the styles never quite clicked, with constant grabbing disrupting the contest.
The officials had it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my greatest performance but what do champions do? They win," Buatsi stated in the arena as he was faced negative reactions from the spectators.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts believed the challenger, thirty-one, should have been awarded the victory but the Derbyshire fighter experienced the second setback of his 28-fight career.
The Champion Earns Triumph in Slow Encounter
Buatsi's progress has slowed in recent times through management issues and extended periods of inactivity.
Earlier forecast as the future of UK boxing, he now is positioned trying to realize his potential - maybe in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter ready to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in two years ago after damaging his hand during the bout, was making just his second fight at the higher weight class.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who worked behind a solid defense to barely take the cleaner periods.
The audience's tolerance diminished because of regular grabbing, with the challenger falling to the floor several times during clinches.
The underdog finished the stronger, but several stanzas were difficult to score. Although, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi advances, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Homage Inspires Manchester Derby Fight
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a terrific all-Manchester contest to capture the European light heavyweight belt.
The arena grew hushed for a profoundly moving tribute ahead of the contest as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who died 46 years old in recently.
It was the first major televised match card in the urban center since Hatton's passing. As the concluding sound disappeared, the stillness became a shout of "only one Hatton".
The fighting inside the ropes reflected the emotion. The eventual winner was floored by a powerful punch flush on the jaw in the initial stage, getting up unsteadily to his feet on the eight count.
But the Manchester United-red shorts of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was injured early, heavy swelling forming under his face as the fighters traded hard punches.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over 12 rounds.
The victor edged the verdict with official scores of 114-114, 115-113 and clearer margin, but each competitor were determined and courageous, all the qualities their idol Hatton represented.
