McClaren admits his side "wasted" a game in their battle to avoid relegation after a 3-0 defeat at Everton.
Two late Ross Barkley penalties, the second of which saw substitute Jamaal Lascelles sent off, followed a first-half Aaron Lennon strike as the Toffees recorded their first home league win since November.
Defeat, which also saw the Magpies lose defenders Chancel Mbemba and Paul Dummett and midfielder Henri Saivet to injuries inside 55 minutes, left Newcastle two points from safety.
McClaren said he had spoken to his players about making the most of every one of their remaining fixtures but felt they had thrown this one away.
"We have said you can't waste a game now and that's what we've done," he said.
"We felt we could come here and get a result but we had to come here and perform at our level and there were a lot of players below that level and that is why we lost.
"Some of them know they have fallen short. That is not what we have come to expect and you can't afford to do that with games ebbing away.
"A few players who did not play at the level and didn't show their quality which is why we lost.
"In terms of injuries and performance that is not our level of performance which we have been producing."
The nature with which they conceded two penalties in the last two minutes raised questions about the players' attitude but McClaren insisted there was nothing wrong in that department.
"The attitude has been a different class the last couple of months," he said.
"We had bad results against Leicester and Crystal Palace (in November) where that (attitude) was questioned but since then it has been excellent.
"I felt it was disappointing we returned to that level and that is unacceptable."