O'Shea told MUTV:
"The manager didn't have to say too much in the dressing room afterwards because everybody's head was hanging.
"Portsmouth took their one real chance and made it pay. The goal gave them an extra yard.
"We are just not making chances. We let our standards slip against Leicester last Tuesday, even though we won, and we let it happen again at Portsmouth - certainly compared to the Arsenal FA Cup semi final.
"It's a big disappointment for the fans especially when they travel so far and we don't even score. They deserve more, but we now have Charlton to think about at our place on Tuesday to make up for it.
"We've given Chelsea an although they still have to come to our place. We certainly don't want to have to pre-qualify with a summer tour to America and a pre-season tournament at Old Trafford planned.
"And we don't want to be stumbling into the FA Cup Final."