The 26-strong Irish squad trained earlier today at the Tatsumi Rugby grounds in central Tokyo. The temperatures again hit the high 30s, with preparation stepped up for this weekend's clash. Japanese officials are already predicting a "full house" at the 22,000-capacity Prince Chichibu Memorial ground.
Humphreys, 33, who took a blow to the face and was forced to retire from last Sunday's 44-12 defeat of the Cherry Blossoms after 21 minutes, showed no ill effects from the facial injury in training.
Humphreys said of the incident: "I took a knock to the face when making a tackle and was in a bit of discomfort following it.
"The Irish medical staff and Japanese staff at the stadium looked after me very well and after various scans and X-rays, nothing was found to be broken.
"The swelling has gone down on my lip and I was feeling happy enough to return to training today. I am feeling fine and should be available for selection for the second Test."
O'Donovan commented: "I think our performance on Saturday was good by the fact that we did not concede anything to the Japanese despite being under pressure in difficult conditions and having so many new faces in the team.
"We would be expecting another competitive game on Sunday and our defence and attack will have to be of the highest order to win.
"We will be playing in similar temperatures to last week and in a much tighter stadium, which will creating a good atmosphere."
The Limerick man added: "There were a number of good individual performances and training has gone well today, with guys still putting the effort in showing that there is plenty of competition for places."
Young backs Kieran Lewis (Leinster) and Conor McPhillips (Connacht) will be champing at the bit to be involved, as the only members of the travelling party yet to make their international debuts.
Munster back row pair David Wallace and Denis Leamy can also expect to be make the matchday squad of 22, having missed out in Osaka.