The 28-year-old, who passed a late fitness test on his back injury prior to the 2-0 defeat at St Mary's, admits it was the only positive on an otherwise bleak day.
Ruiz played for 81 minutes against the high-flying Saints, and he hopes he can atone for the loss at the King Power Stadium.
The Costa Rica international reckons victory at Leicester is vital ahead of next weekend's clash with Manchester United.
"I still have some problems with my back and we're working on that," Ruiz said.
"It's getting better - hopefully in the next couple of weeks it will be 100 per cent.
"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."