Shaun Bielby reportedly flew into a rage after catching his partner of 10 years naked in bed with another Celtic fan.
He then launched into a vicious attack against alleged love rat Alan Davey, 57, smacking him in the head with a large piece of wood.
The assault left Davey with a wound on his forehead which needed nine stitches.
He was also left “covered in blood” and “looking like a zombie”, Paisley Sheriff Court heard.
Davey and Bielby had been in a pub earlier in the evening with Davey’s girlfriend Yvonne Kearney, celebrating the Old Firm side’s second consecutive domestic treble on May 19 last year.
“I was woken up by what I now realise was a blow to the head”
Bielby left earlier to go home, leaving Davey and Kearney together in the bar.
The boozed-up pair then went back to Davey’s brother Patrick’s home in Paisley and ended up in bed.
When Bielby came to the house looking for his girlfriend, he discovered the pair naked in bed together.
Grabbing a piece of wood that was lying in the hall, he used it to batter Davey.
Bielby appeared in court on Wednesday, after earlier pleading guilty to a charge of leaving Davey scarred for life and seriously hurt by assaulting him with “an unknown implement”.
But he denied assaulting his girlfriend by hitting her with the same weapon.
Davey said he was sleeping in bed naked and woke up to Bielby’s attack.
He explained: “I was woken up by what I now realise was a blow to the head.
“Then the next four to five minutes was a series of blows to the head, the body, the arm and the back.”
He said he noticed he “had blood running down” his face, and said Yvonne was also in distress.
When asked if the pair had been doing something they shouldn’t have, Davey, who is married, replied: “I don’t recall but I can’t rule it out because of the alcohol”.
Sheriff Seith Ireland called for background reports to be prepared and deferred sentence for Bielby’s assault until next month.
Addressing Bielby, Ireland said: “It may well be there are some extenuating circumstances which your solicitor will ask me to take into consideration.
“You seriously assaulted this man and left him with permanent disfigurement — that is a serious matter.”