British light heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a competitive ten-round fight at the boxing arena.
But Parker found success, connecting with powerful individual punches in a fight where the styles never quite clicked, with constant grabbing disrupting the contest.
Two judges had it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest performance but how do champions respond? They succeed," Buatsi said in the ring as he was greeted with disapproval from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts believed the challenger, thirty-one, should have been awarded the win but the Derbyshire fighter experienced the second setback of his 28-match career.
The Champion Earns Triumph in Dull Match
His career development has stalled in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the next star of British boxing, he now stands trying to achieve expectations - maybe in what feels like a last chance - under new promotional backing.
He began cautiously, with neither fighter ready to take risks in a calculated early stages.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was competing in only his second fight at the higher weight class.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who operated with a solid defense to edge the better stanzas.
The audience's tolerance diminished because of constant clinching, with Parker slipping to the canvas various instances during close exchanges.
The underdog finished the stronger, but many rounds were hard to judge. However, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
The victor moves on, but with the likes of hard hitter David Benavidez, and Russian champion Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Homage Inspires Manchester Derby Contest
In the secondary fight, Arthur defeated Rea by split-decision in a terrific all-Manchester fight to capture the European light-heavyweight belt.
The arena fell silent for a deeply poignant memorial prior to the bout as a bell ceremony tolled in memory of local hero Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the concluding sound faded, the silence gave way to a roar of "there's only one Hatton".
The competition during the bout reflected the feeling. Arthur was knocked down by a powerful punch squarely on the chin in the second round, getting up unsteadily to his position on the eight count.
But the red-colored attire of the fighter soon advanced again. On the opposite side, the opponent - in opposing blue - was injured soon, heavy swelling forming under his orbital area as the duo exchanged punishing blows.
Rea, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the pace hardly diminished over the full distance.
Arthur narrowly won the decision with scoring tallies of even score, close margin and 115-112, but each competitor were determined and full of heart, everything their idol Hatton stood for.
