The Hammers were beaten 2-1 at Elland Road, leaving them 19th on the Premier League table.
Afterwards, Nuno said: "There are many problems in our club unfortunately. It is not up to us to hide ourselves behind the problems. Everyone has to be alive and to do much more and be in the right position.
"We were not dealing with our defensive situations and I felt like we needed a striker to hold the ball, so maybe that's not the greatest from me.
"These kind of mistakes are unacceptable in the Premier League."
Nuno also said: "There is quality there, there is time, but nothing will happen if we don't change.
"We must change our attitude, we must change the way we approach things, we must commit ourselves better, prepare better, work harder.
"All the things - that is the reality. We don't expect things to change by themselves. Realising we have time can be a mistake if we don't change things around quickly."
On the match, he added: "It's a simple explanation. We didn't deal with the problems. We didn't deal with many things. We have to work hard and be much better. I feel the players want to, but they switch off in many moments."