Fonte joined top-flight rivals West Ham in an £8 million move on Friday, with the Portugal defender having been dropped by Southampton manager Claude Puel earlier this month after he handed in a transfer request.
Phillips ends Leicester's home run
But the Saints, no stranger to seeing their players leave for more lucrative offers at 'bigger' English clubs during the transfer window, shrugged off Fonte's loss to end a run of four straight Premier League defeats in style.
For Leicester, this latest loss was their 11th in 22 league games so far this season and left them just five points above the relegation zone.
Southampton were 2-0 up before half-time thanks to goals from James Ward-Prowse and Jay Rodriguez.
The south coast side wrapped up just their seventh league win of the current campaign four minutes from time when Dusan Tadic scored from the penalty spot.
Bournemouth make sure Leicester's league revival is short-lived
Victory was just the boost Southampton, now 11th in the table, needed ahead of their League Cup semi-final second leg clash against Liverpool on Wednesday, with the Saints taking a 1-0 lead to Anfield.
"The first goal in any game is important," midfielder Ward-Prowse told Sky Sports. "After the bad run we have been on this is a turning point in the season.
"Wednesday is massive. It (Anfield) is a tough place to go but we can take confidence from today and how we played against them in the first leg and hopefully we can get to Wembley."
Meanwhile Leicester manager Claudio Ranieri blamed himself for his side's defeat and promised to revert to the formation that proved so successful for the Foxes during their shock title triumph last season.
"I think the last matches I changed shape to try to help my players play better but maybe I made mistakes," he said. "Against Chelsea we played three at the back and today we wanted to play with a diamond.
“The players are used to 4-4-1-1 and know the positions but I made a mistake. I think it is much better to give them what they know well."
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ