British light heavyweight Joshua Buatsi earned a win yet underwhelming beginning to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a competitive 10-round fight at the boxing arena.
But Parker landed significant shots, landing some telling single shots in a contest where the approaches never quite clicked, with frequent clinching marring the fight.
The officials had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my best display but what do champions do? They succeed," Buatsi 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 last winter - to secure a 20th pro success.
Some ringside observers thought Parker, 31, deserved the victory but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
The Champion Earns Triumph in Dull Encounter
Buatsi's progress has plateaued in recent years through contract problems and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to achieve expectations - maybe in what feels like a last chance - under recent support.
He started carefully, with the two boxers ready to overcommit in a calculated initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was making just his second bout at light heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who worked behind a tight guard to barely take the cleaner periods.
The crowd's patience diminished during regular grabbing, with the challenger slipping to the canvas multiple occasions during close exchanges.
The underdog closed well, but many rounds were challenging to assess. However, ex-champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the caliber of hard hitter David Benavidez, and Russian star Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a excellent Manchester vs Manchester contest to capture the European light heavyweight championship.
The stadium grew hushed for a extremely emotional homage ahead of the contest as 10 bells rang in memory of local hero Ricky Hatton, who died 46 years old in last month.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the final chime ended, the stillness transitioned to a roar of "there's just one Hatton".
The fighting during the bout reflected the emotion. The eventual winner was floored by a powerful punch flush on the face in the second round, standing uncertainly to his feet on the eight count.
But the traditional red attire of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was damaged early, heavy swelling forming under his eye as the duo landed powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over the full distance.
The victor narrowly won the decision with scoring tallies of even score, 115-113 and 115-112, but both fighters were determined and full of heart, exactly what their idol Hatton embodied.
