Alexander, who has communicated with Wighton via text, told AAP that NSW coach Brad Fittler spoke on Sunday to the 30-year-old, who said he was stepping down to "focus on club and family".
Wighton has played seven Tests for Australia and 10 Origin games for NSW in multiple positions.
"He has been a big part of the NSW side for the last four years so it is a blow. Jack is a great Origin player," Alexander said.
Alexander said Wighton, aside from his skill levels, brought "a toughness that is unique" and would be missed.
"Jack's efforts when he has been on the field have been extraordinary," Alexander said.
"When you mention Origin, Jack's name belongs with that style of football ... tough, unrelenting, give anything for his teammates.
"(Wighton's retirement) is disappointing ... but it is his decision."
Alexander said Wighton would be hard to replace but the return of Latrell Mitchell and Tom Trbojevic after missing last year's series was a plus.
"There is still a lot of footy to be played before the side is picked," he said.
"Tom and Latrell have made returns to the game after being injured through last year's series but Jack's impact wasn't just at centre.
"He did play centre but he had great utility value. He could play anywhere."
South Sydney's Blues representatives Damien Cook and Cameron Murry responded to the Wighton news on Monday.
"It's a bit of a surprise to me," Murray said.
"(Origin) takes a pretty big chunk out of you. He obviously wants to give his all in clubland.
"So I'm happy for Jackie for making the right decision for himself and his family."
Cook hailed Wighton as "a great Origin player".
"He puts his team first and does what the team needs and puts his body on the line," Cook said.
"I was very honoured to get the chance to play alongside Jack."
Off-contract Wighton is in the midst of deciding his future for 2024 and beyond after informing the Raiders he would test the market.
Cook said he would be welcome at the Rabbitohs.
"Someone like Jack Wighton, you'd find a position for them anywhere in the team, he's just a footballer who wants to win games," Cook said.
"Trell (Latrell Mitchell) took all the money so I don't know if there's any left for his mate Jack."
The centre position is wide open for the Blues and Cook said his Rabbitohs teammate Campbell Graham fitted the bill.
"Defensively he's got to be the best centre in the game," Cook said.
"In Origin you are coming off your line most of the game and it is all about yardage carries. We depend on him here, that's what he does.
"There's no doubt if he got his opportunity he'd do the jersey proud."
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