Simon Grayson was left angry after watching his Sunderland side dismantled by Celtic - admitting it was a big "eye opener" ahead of Friday's Derby County Championship clash.
The Black Cats were thoroughly outclassed and outplayed by an under-strength Celtic side at the Stadium of Light, a showpiece friendly to celebrate the stadium's 20-year anniversary.
There was nothing to celebrate though after Callum McGregor bagged a hat-trick with Jonny Hayes and Stuart Armstrong also on the scoresheet to leave Sunderland floored.
Grayson - who insisted the team selected to face Celtic in the final friendly wasn't necessarily his strongest team - was hugely disappointed by the display.
Grayson said: "It is not ideal, it is an eye opener for everybody - just when you think you are going along in good fashion you suffer a result like that.
"It is hugely disappointing. There has been a lot of factors over the past few weeks that have been quite good in terms of how we played but today we didn't do the basics right.
"We didn't work hard enough as a team, too many bad decisions, too many errors, and obviously when you make those decisions against a really strong team like Celtic they will punish you.
"You can't make those decisions in any level of football as you will get punished. It is a wake up call to all of us.
"It was a day when anything that could go wrong, did go wrong. Hopefully it won't happen again.
"A bad mistake for the first goal, then missing chances, a penalty, then a penalty awarded that is outside of the box, a deflected fifth goal.
"Anything that could go wrong, did go wrong, but that is only papering over the cracks. We have had a really bad day and we need to get better.
"That starts Sunday morning when we analyse what happened today."
Grayson named all six of his summer signings in the starting line-up with Jason Steele, Tyias Browning, Brendan Galloway, Aiden McGeady, James Vaughan and Lewis Grabban making their Stadium of Light debuts.
When asked if he was doubly concerned given this was his strongest available side ahead of the Championship opener against Derby County, Grayson said: "After today it is not.
"I said all along this last game isn't a rehearsal for the game on Friday, people can play themselves into the team and out of the team.
"We will sit down in the morning with the group and go through what we need to do better."