Leinster's Rob Kearney has emerged as a doubt for Ireland's World Cup opener against Scotland on Sunday.
The 33-year-old is believed to have picked up a calf injury in training earlier today.
If he is out it will be another injury blow to Joe Schmidt's plans.
Ireland could be without three of their regular backline against the Scots, with Robbie Henshaw definitely out of the Yokohama showdown and Keith Earls expected to miss out.
Kearney, who was in line for his 93rd Ireland cap, could also be in danger of missing the clash with hosts Japan next week.
Schmidt will turn to Jordan Larmour or Andrew Conway to fill in for the vastly experienced Louthman, who had hit top form by the end of the warm-up series, if he misses out.
Henshaw has remained with the squad after a scan confirmed his hamstring injury wasn't serious but will miss at least one game.
Earls, meanwhile, could resume full training on Wednesday after a knee problem but that could be too late for inclusion in Schmidt's plans this weekend.
