PSV were beaten 3-0 in Spain in the first leg of their knockout stage playoff last week and have a mountain to climb in the return at the Philips Stadium if they are to get themselves back into contention.
Sevilla struck just before halftime and added two quick goals after the break for an emphatic victory but Van Nistelrooy said he drew heart from PSV's first half showing.
"That is the highest level we have achieved this year. Keeping that up is the next step. We want to put pressure on Sevilla. We have no other option tomorrow," said the former Dutch international striker, back from illness after missing PSV's match in the Eredivisie against Utrecht on Sunday.
"Getting back up to the level is what I call development. Especially with five to six young players, who are between 19 and 23-years-old. Mistakes are immediately punished at that level. That is frustrating and disappointing, but how you deal with it is also picking up vital experience. You also have to learn that as a team," he told a news conference on Wednesday.
PSV will still be up against it as they face elimination from European club competition and, combined with a six-point deficit on leaders Feyenoord in the Dutch league, could return a disappointing season – Van Nistelrooy's first in charge.
"Even after selling our most important players, I said that we are sticking to our ambitions, but the chance of achieving your goals is of course smaller," the 46-year-old said.
The club sold Cody Gakpo to Liverpool and Noni Madueke to Chelsea in January. "Everyone knows what happened in the transfer window and why. But we continue with the young boys and we will develop them. That's the goal. We'll see where we are at the end of the season," added Van Nistelrooy.