The match was played under baking sun in the Welsh capital but McGuinness said the conditions had little to do with the result.
"I don't think it was the weather. It was the attitude of our players - they showed a lack of character. It was a big disappointment and those performances put doubts in everyone's minds.
"They'd better improve. If they continue folding as easily as they did in the second half then they won't have a career in football.
"They need to know [the levels they showed today] are not the standards required to be a professional footballer... certainly not at Manchester United."