Saints won 2-1 to book a Wembley semifinal place.
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Shearer, who began his pro career with Saints, said on BBC Sport: "This is not a fluke.
"Southampton deserve their trip to Wembley.
"They were absolutely magnificent and the job that Tonda Eckert has done, what a night for them. They still had the belief that they could go on and win this game and they did. This is not a fluke. Thoroughly deserved.
"What a job that Eckert has done, the belief that he has given this football club from where they were to where they are now."