Manchester United's whizkids are reeling after being dumped out of the FA Youth Cup by Carlise United.
"We made basic mistakes in the build-up to Carlisle's goals. We played some good football after that, but we didn't carry that into the second half. We didn't test the goalkeeper enough.
"We had loads of possession and even played some good passing football, but we were too narrow. We didn't use the width well enough and didn't get enough crosses in."
McGuinness says his young players have passed up a big chance to impress on youth football's biggest stage.
"The defeat puts us back two steps," he added. "We've missed an opportunity to do well in this competition. The players now have to lift themselves. It's a big defeat and at this club sometimes that's a burden you have to carry."