Saints were beaten 2-0 by the Foxes as Jamie Vardy and Jordan Ayew struck the goals in this battle for the relegated clubs.
Rusk later said, "I actually think we started the game okay. The first goal, I've not watched the game back but it might have been against the run of play.
"I thought we were arriving in some pretty dangerous areas ourselves, and the first goal just seemed to knock the confidence out of the team.
"Maybe it was as a result of the season as a whole. It seemed to be a step too far, psychologically perhaps, the performance just ran away from us."
"We expected a difficult game"
Rusk was also asked if the players were giving their all in these final games of the season.
He also said, "There's a reality to confidence levels, we can try and affect it as much as we can, but the reality is we had seven games to go and we're working together as a group that have won two games all season.
"We are away from home in the Premier League against a team with seven more points than us. I'm not entirely certain of the exact points tally.
"I expected a difficult game. I expected a difficult game as can happen in the Premier League and with where we're at, at the moment.
"It went against us and we struggled to maintain performance from there but I think there's a lot of things in the mix.
"It's very hard for me to sort of like isolate it down to one thing and yeah, it turned into a really difficult day."