Jose Mourinho swooped in last week to sign Matic from Chelsea in a deal worth £40m.
Matic was surplus to requirements at Stamford Bridge following Tiemoue Bakayoko's arrival from Monaco.
Antonio Conte recently hinted that his departure was not his decision and has already aired his frustration at Chelsea's current transfer activity stating he can't afford to sell anymore players.
But the 29-year-old has proved a valuable asset to Chelsea over the years and was a key part of the 2015 title winning side.
And Liverpool legend Souness was shocked to see Chelsea allow the midfielder leave to join their title rivals.
"I have some criticism of the Serb — I would like to see him become more of a box-to-box player, contributing more goals (remember that brilliant strike against Tottenham in the FA Cup semi-final)," Souness told The Sunday Times.
"But he has a great workrate, doesn't give the ball away and was an integral part of two title-winning teams.
"At 29 he is at his peak and I would not have sold him.
"Or, if I did want to move him on, it would not have been to Old Trafford because it only strengthens one of Chelsea's rivals.
"A stronger United makes life harder for Chelsea and I expect Manchester City to do better this season."
