UK light heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a competitive ten-round contest at the Manchester venue.
But Parker landed significant shots, delivering effective clean blows in a match where the approaches never quite clicked, with constant grabbing spoiling the fight.
The officials had it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my finest display but what do champions do? They prevail," the victor stated in the ringside as he was faced boos from the spectators.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts felt Parker, 31, deserved the decision but the British boxer endured the second loss of his 28-bout professional journey.
The Champion Secures Win in Slow Affair
The fighter's advancement has plateaued in recent times through contract problems and long spells of inactivity.
Once tipped as the future of UK boxing, he now is positioned trying to achieve expectations - possibly in what seems like a final roll of the dice - under recent support.
He began cautiously, with both competitors prepared to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand during the bout, was competing in only his second appearance at light heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for Buatsi, who operated with a protective stance to narrowly win the cleaner rounds.
The spectators' acceptance wore thin during frequent holding, with Parker falling to the floor several times during clinches.
Parker ended stronger, but many rounds were difficult to score. However, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher David Benavidez, and Russian star Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Memorial Motivates Manchester Derby Battle
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a excellent all-Manchester fight to secure the European light-heavyweight title.
The venue fell silent for a extremely emotional memorial ahead of the contest as a bell ceremony tolled in remembrance of the city's native Hatton, who died at age 46 in last month.
It was the first major televised fight night in the urban center since Hatton's death. As the last bell ended, the stillness gave way to a chant of "only one Ricky Hatton".
The fighting within the ring matched the sentiment. Arthur was sent to the canvas by a powerful punch squarely on the face in the second round, rising shakily to his position on the referee's count.
But the red-colored attire of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was marked up quickly, noticeable damage developing under his face as the fighters exchanged punishing blows.
The challenger, 27, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the intensity hardly diminished over twelve rounds.
Arthur edged the verdict with judges scorecards of draw, close margin and clearer margin, but each competitor were persistent and full of heart, everything their hero Hatton embodied.
