Claude Puel says he is not concerned about the boos at full-time for his Leicester team as 10-man Southampton left the King Power Stadium with three points.
Long scored in first-half injury-time just two minutes after Yan Valery was sent off for two bookable offences.
Wilfred Ndidi's consolation failed to rally Leicester as Ralph Hasenhuttl's Saints moved out of the Premier League relegation zone.
The Foxes were booed off with at least one 'Puel Out' sign spotted but the Frenchman will ignore the critics.
Puel said: "The fans are disappointed like us and it's a normal thing, it's not my concern, I have to keep my focus and concentrate on the team.
"It's a big disappointment, we wanted a good response after our cup game. The players were focused, perhaps a little nervous, and we showed this in the first half.
"We wanted to perform, we wanted to have a reaction and our concentration was perhaps too much. It was a difficult scenario.
"We didn't manage the first half well. It would have been good to have come back without the second goal but it was harsh and it was a bad moment.
"We played with desire and intensity in the second half but we didn't perform enough."