UK light-heavyweight Joshua Buatsi made a winning yet disappointing start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a close ten-round contest at the boxing arena.
But Parker found success, delivering effective individual punches in a match where the styles failed to mesh, with constant grabbing disrupting the fight.
The officials judged it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They prevail," the victor stated in the ring as he was met by negative reactions from the spectators.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts felt Parker, thirty-one, merited the decision but the contender suffered the second setback of his 28-bout career.
Buatsi Claims Triumph in Uneventful Affair
His career development has plateaued in recent years through contract problems and extended periods of inactivity.
Earlier forecast as the future of UK boxing, he now stands trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under recent support.
He began cautiously, with the two boxers prepared to take risks in a cagey early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after suffering a hand injury mid-fight, was participating in his second appearance at light heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for Buatsi, who utilized a tight guard to barely take the cleaner rounds.
The crowd's patience diminished during constant clinching, with Parker falling to the floor several times during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. However, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Hatton Memorial Motivates Manchester Derby Fight
In the co-main event, Arthur beat Rea by split-decision in a terrific local rivalry battle to win the European light-heavyweight belt.
The stadium fell silent for a extremely emotional tribute prior to the bout as ten bells tolled in honor of the city's native Hatton, who died 46 years old in recently.
It was the premier televised boxing event in the urban center since Hatton's death. As the last bell disappeared, the stillness became a shout of "there's only one Ricky Hatton".
The competition within the ring matched the feeling. Arthur was floored by a powerful punch directly to the chin in the initial stage, standing uncertainly to his feet on the referee's count.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in rival City blue - was damaged early, noticeable damage developing under his eye as the pair exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity hardly diminished over 12 rounds.
The victor barely secured the decision with scoring tallies of even score, 115-113 and wider gap, but the two athletes were relentless and full of heart, all the qualities their inspiration the legend embodied.
