The Saints confirmed on Monday that they had parted company with the Welshman after eight months in charge, with the club sitting 18th in the table.
After Hughes steered Southampton to safety in the 2017/18 campaign, he was rewarded with a three-year contract.
However, former RB Leipzig head coach Ralph Hasenhuttl has now taken the reins, being given a two-and-a-half-year deal at St Mary's.
Shearer, who came through the ranks at Southampton and scored 43 senior goals for them between 1988 and 1992 (Soccerbase), was not impressed with how the club has acted.
"To make the decision after just thirteen games of the season – thirteen games after giving him a three year contract we should remember – that he is no longer good enough, just goes to show what football is like now," he wrote in Coral. "It happens time and time again and I suppose we shouldn’t be surprised anymore."
In addition, the 48-year-old gave his thoughts on what may have caused the rot to set in at St Mary's, and he seems to feel that it's something more at board level than on the pitch itself.
"Year after year Southampton have sold their best players and there is a feeling that it is coming back to haunt them now," Shearer continued.
"Because going out and constantly buying new players that fit your club is the hardest part of football. They’re starting to come unstuck and it’s perhaps not that surprising when you consider the top class players that they have sold, and how hard it is to keep replacing them."
Hasenhuttl led Leipzig to a second-place finish in the Bundesliga in his first season in charge but left at the end of the 2017/18 campaign after failing to agree a contract extension.