The former Hoops star insists his fellow defender has stepped up in the last 12 months to become a priceless part of Brendan Rodgers' side.
But Mulgrew reckons the Irishman still doesn't get the credit he deserves - simply because he never arrived in a multi-million pound deal like many others around about him.
Scales was recruited from Shamrock Rovers for £500,000 three years ago and spent his second season out on loan at Aberdeen.
But having formed a solid partnership with Cameron. Carter-Vickers under Rodgers the 25-year-old has become a first pick for Celtic and Ireland and was rewarded with a new deal at the start of this year.
Rodgers admitted after last month's Old Firm win his side would be lost without Scales.
And Mulgrew, who had two spells at Parkhead and won five league titles, said: "Liam has been absolutely brilliant.
Brendan has openly said he doesn't know where he would be without him.
"I think he was nearly out of the door and then he stepped up to the plate.
"Because he's not a big or foreign name, sometimes people are just waiting for him to make a mistake and looking at the things he doesn't do, rather than the actual stuff he does bring.
"If they'd signed him for six, seven or eight million, the place would be raving about him and he'd get a lot more credit for what he's done."
Scales spent a full year at Pittodrie on loan from Celtic after being farmed out by Ange Postecoglou.
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