Robbie Henshaw has emerged as a major doubt for Ireland's opening Six Nations clash against England in Dublin next year.
The Leinster centre picked up a hamstring injury during the warm-up session prior to Ireland's game against Argentina last week.
Initially ruled out for six weeks and pegged for a January return, a revised assessment has left him struggling to be fit for the start of February.
Bad news for Joe Schmidt as he goes about defending the Six Nations title and backing up a Grand Slam.
And for Leo Cullen who will have to negotiate three rounds of Heineken Champions Cup and three Guinness PRO14 interpros without the classy midfielder.
Cullen may also have to do without out-half Jonathan Sexton for this Saturday's European encounter with Bath at the Aviva Stadium.
Sexton was withdrawn with a calf injury last weekend when the same sides met at the Rec.
"Johnny got a bang on his calf so we're not treating it as too serious,” said senior Leinster coach Stuart Lancaster.
“That was the reason he came off, it wasn't any more than that.
"But obviously we need to give it time to settle and hopefully he'll recover for the weekend, but we can't say anymore at the moment.
"We want to give him as much time as possible, but if not then Ross (Byrne) will be ready.
"Ross is a quality player. He was outstanding against Dragons (two weekends ago) and in my mind more than earned his international call-up.
"And he proved that when he's played in the autumn series as well.
"We've every confidence in Ross if Johnny's not available."
