Former world super-bantamweight champion Scott Quigg hinted at retirement after he was outclassed by Jono Carroll in Manchester.
The Briton, 31, hoped to become a two-weight world champion after stepping up to super-featherweight to face Carroll.
But the Irish southpaw, 27, upset the home crowd by dominating to win by technical knockout in the 11th round.
Quigg said: "This was a must-win fight. On this performance, I don't know what's left. If this is it, thank you."
He added: "I gave it everything tonight and unfortunately it wasn't there.
"Taking nothing away from him, the better man on the night. From after the fourth round I just didn't feel like it was there. I was chasing it."
The Bury fighter was fighting for the first time in 17 months and suffered just a third defeat in his 40-fight career.
Carroll has improved his record to 18-1-1, with his solitary loss coming when he challenged Tevin Farmer for the IBF super-featherweight title last March.
After recovering with a win over Eleazar Valenzuela last August, a close contest was expected with Quigg, who held the WBA super-bantamweight belt before losing to Northern Ireland's Carl Frampton in 2016.
But after watching Carroll dominate, Quigg's corner threw the towel in during the 11th round, bringing an end to just his second fight since losing to Oscar Valdez for the WBO featherweight title in March 2018.
After the fight, Quigg's trainer Joe Gallagher came in for some criticism on social media for not stopping his man sooner, while ringside analysts were also unhappy.
Speaking on Sky Sports, former two-weight world champion Frampton said his main emotion watching the fight was "anger", adding: "The towel should have come in in my opinion maybe four rounds before it did. I was looking at the corner and shouting from my seat."
Trainer Dave Coldwell added: "I found it so frustrating to watch. He was never in the fight; he just got beat up."
Elsewhere on the undercard in Manchester, Hughie Fury got back to winning ways after his loss to Alexander Povetkin in August.
The British heavyweight, 25, improved to a 24-3 record with a third-round stoppage of the Czech Republic's Pavel Sour.
The pair were scheduled to meet in Monte Carlo last November but Fury had to pull out due to a virus.