The Blues caretaker boss chose Richard Kingson to keep goal against in-form Pompey ahead of regular number one Maik Taylor and inevitably it was a goalkeeping error which led to the visitors taking a one goal lead at St Andrew's.
"You've got to make decisions and play your best players," he said after the 2-0 loss.
"Maik has been away on international duty while Richard has been looking good and deserved his opportunity.
"It was a poor goal and no-one is more disappointed than him. At 0-0, it was not a bad result for us against a top side."
On his possibilities of becoming successor to Steve Bruce, he added: "I am employed by the club and will give it everything I've got.
"I've not been offered the job and I'll just carry on what I'm doing.
"My main focus now is analysing what went wrong today and then we will start again on Monday preparing for next weekend's game at Tottenham.
"I enjoyed it, although obviously I would have preferred if we had won."