The Dutch champions and league leaders will protect a 1-0 advantage next week after Gilberto's blunder gifted Jefferson Farfan the winner in the first leg of the last-16 clash.
It was the second defeat in a row since the Carling Cup victory over Chelsea and dented Ramos' hopes of winning the UEFA Cup again after lifting the trophy with Sevilla in the last two years.
"You could say that it was a reality check," he said. "Against Birmingham at the weekend and now PSV we haven't played as well as we could have. We have lowered our tempo."
"The barometer is very low," he said. "If we want to go through we have to improve.
"It's true, we had our best 11 players out there and were disappointed as PSV were better."
The night got worse for Ramos when Jermaine Jenas twisted an ankle in the second half and was carried off on a stretcher.
PSV could have extended their lead but Robinson pulled off a stunning save to deny Farfan.
"Of course I didn't like the result but we still have a second leg and when you lose by one goal it's not too bad," Ramos said. "But we have to play much better game.
"After this it's going to be very difficult and we are going to have to improve if we want to go through.
"They played better than us, they are very good - first in the Dutch league with a lost of points advantage, and have come from the Champions League."
