Steve McClaren was left angry, frustrated and disappointed after his Newcastle United side produced an "unacceptable" display in Wednesday's 3-0 defeat at Everton.
The Magpies struggled to assert themselves at Goodison Park as the Merseysiders claimed a first home Premier League win since November.
Aaron Lennon broke the deadlock midway through the first half before two penalties from Ross Barkley - making his 100th Premier League appearance - wrapped up the points for Roberto Martinez's side.
"Tonight we went back to what we were three or four months ago and it wasn't what we came here to do and it wasn't acceptable," he told Sky Sports.
"You could see in the first half that the Everton players and crowd were nervous and we didn't take advantage of it. We weren't positive or aggressive enough.
"Lennon's goal was a sloppy goal and gave them a lift. We changed things around in the second, where we were unfortunate to have injuries to Chancel Mbemba and Paul Dummett so we had to make changes.
"We had a go in the second half but it wasn't enough, we ought to be performing a lot better than that, fighting for each other, working for each other and playing good football.
"There were too many players below par and we need a reaction at home on Saturday, in front of 52,000 we have to perform."
Newcastle, who remain 18th in the Premier League following Wednesday's loss, welcome West Brom to St James' Park as the North East club look to haul themselves clear of the relegation zone.
Asked whether he was confident his side could bounce back on Saturday, McClaren added: "We've had it before where you have bad games and bad halves and my players have come back and proven they can do it.
"We will need the support of the crowd but we have to make a commitment to really go out there on Saturday and put this one behind us and get a result."
Untimely defensive injuries at Goodison Park forced McClaren's hand to an extent, with Mbemba and Dummett forced off early on.
As a result, January recruit Seydou Doumbia - signed from CSKA Moscow - was not handed his debut for the club.
However, McClaren was keen to add the club are still assessing the Ivorian before plunging him into first-team action.
"We'll have to assess that [Doumbia] over the next couple of days as they were in a winter break in Russia," McClaren said.
"He has been training but last played in December so it's a bit difficult, we don't want to put too much pressure on him. We just have to ease him in, as with the other new boys."