British light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a judges' verdict in a close ten-round contest at the Co-op Live Arena.
But the challenger found success, landing some telling individual punches in a match where the techniques never quite clicked, with frequent clinching marring the action.
The officials judged it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my best showing but what do champions do? They win," Buatsi commented in the ring as he was faced disapproval from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers felt Parker, 31, should have been awarded the win but the Derbyshire fighter suffered the second loss of his 28-match professional journey.
Buatsi Secures Triumph in Dull Affair
Buatsi's progress has plateaued in recent years through contract problems and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to realize his potential - perhaps in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors prepared to overcommit in a tentative opening few rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who utilized a protective stance to narrowly win the cleaner periods.
The crowd's patience wore thin amid constant clinching, with the challenger falling to the floor several times during close exchanges.
The underdog finished the stronger, but many rounds were challenging to assess. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor advances, but with the presence of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a terrific all-Manchester contest to capture the European light heavyweight title.
The venue grew hushed for a extremely emotional tribute before the fight as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the concluding sound ended, the stillness gave way to a chant of "there's just one Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was floored by a left hook directly to the jaw in the early round, standing uncertainly to his feet on the count of eight.
But the red-colored shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in opposing blue - was marked up quickly, heavy swelling forming under his orbital area as the fighters landed powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with effective uppercuts as the pace seldom decreased over twelve rounds.
The victor edged the decision with official scores of 114-114, close margin and clearer margin, but the two athletes were persistent and brave, exactly what their hero Hatton stood for.
