The Cottagers were always second best at the weekend against the in-form Saints, who could have won by more than first-half goals from Rickie Lambert and Jay Rodriguez.
And striker Ruiz knows they need to buck up their ideas if they are to avoid more disappointment, starting at the King Power Stadium.
"Southampton played well," Ruiz told the club's official website.
"We had a plan but once they scored the first goal it was difficult to find a way back into the match. They deserved the win but now we have to pick ourselves up and look towards the next match.
"After winning two games in a row, we wanted to continue that at Southampton. We weren't able to do it and we were all disappointed with the display.
"The most frustrating thing was not testing their goalkeeper. We should have done more but Southampton have only conceded three goals this season and they're very strong defensively.
"They are in good shape and full of confidence but we need to try and forget about it now and concentrate on the next match.
"We've got two games coming up quickly and it will be a tough week. We want to qualify for the next round of the cup and then take on Manchester United in a good frame of mind."
Costa Rica international Ruiz had to pass a late fitness test on his back injury in order to play at St Mary's.
The 28-year-old managed to come through 81 minutes unscathed and hopes to be involved against the Foxes.
He added: "I still have some problems with my back and we're working on that.
"It's getting better - hopefully in the next couple of weeks it will be 100 per cent."
- Sports & Recreation
- Manchester United
- Rickie Lambert
- Martin Jol