The 34-year-old Bath hooker is struggling with a shoulder injury which he suffered during his club's Premiership victory over Saracens three days ago.
Although a scan on his left shoulder on Monday revealed no serious damage, there is inflammation around the joint which must raise a question mark over his fitness to face the Scots on Saturday.
"It is still pretty sore," said Humphreys at the Wales training base.
"But hopefully by Wednesday I will be able to take part in the team session. I will have more physio and rehab, and my goal is to take a full part in the session.
"I am very, very hopeful that I will be 100% right."
If Humphreys is ruled out, and he is likely to make his own decision either way on Wednesday, then Wales coach Steven Hansen is likely to recall Pontypridd hooker Mefin Davies, or promote promising Bridgend youngster Gareth Williams off the bench.
And in Humphreys' absence, the captaincy would almost certainly revert back to Colin Charvis, whom Humphreys replaced as skipper for the Millennium Stadium appointment with England 10 days ago.