Portal said that there are no underlying illnesses or injuries to blame, and that Froome simply "did not have the legs today".
He also recognised that Froome had come into the Giro with the strategy of building his form throughout the race and that his condition was "not as strong as it could have been".
"But we are where we are now," he added, "and we've still got two weeks to go."
Portal said that the team had gone into the Sasso stage feeling confident, and that morale remained high. But there had come a point in the closing kilometres where Froome began to struggle, and the objective of gaining time switched to simply not losing any more.
"It was a tough day. It was just his legs, and that's all," Portal explained. "He couldn't do anything more, and he lost 67 seconds.
"He was there at three kilometres to go, then at two kilometres he lost contact with the top group and then lost more and more time. He had been feeling good in the past few days, and we were feeling confident. But then we had to start concentrating on not losing time."
Portal was adamant that Froome could fight back from where he stands now, in 11th place, 2:27 down on race leader Simon Yates (Mitchelton-Scott).
"It's a big challenge to win the Giro, and we knew it would be difficult. It's been a hard first week, but the war is not lost."
Portal confirmed that two crashes in a week, the first prior to the opening time trial, and then a second on Saturday, had not been beneficial to Froome. But he was also clear that "it could not be used as an excuse".
"His legs aren't as strong as they could be, and he's had the crashes, too. I think the rest day will be very welcome for him now."