Fly-half Priestland and blindside flanker Lydiate have been struggling with respective knee and ankle injuries picked up in European action for the Scarlets and the Dragons the weekend before last, while centre Roberts suffered knee damage in the Blues' pre-Christmas derby with the Dragons.
But Howley says he and head coach Warren Gatland haven't ruled out the star trio being fit to start the tournament, with the next two days being a crucial period in their rehabilitation.
"With Dan Lydiate and Rhys Priestland we are still waiting," said Howley.
"In terms of their participation in training, it was very limited last week. Dan and Rhys probably did about two or three sessions but they were just limited to skills.
"The next 48 hours is pretty critical and we are waiting to see how they are.
"They have come in this morning and they are a little bit sore after the weekend's endeavours but we are obviously hopeful.
"Jamie participated a little bit more than Dan and Rhys but we are still uncertain. He has not played for a long time, I think it's around five or six weeks, so we are, once again, waiting to see how the next 48 hours goes."