Robbie Henshaw is reported to be a fitness concern for Ireland's World Cup opener against Scotland on 22 September because of a hamstring injury.
The Irish Times is among media outlets reporting that the Leinster centre has suffered the injury.
Henshaw, 26, looked set to partner Bundee Aki in the Ireland midfield after the duo impressed in the final warm-up win over Wales last weekend.
Garry Ringrose or Chris Farrell would be in line to replace Henshaw.
If Henshaw's injury puts his entire World Cup participation under threat, Ulster's utility back Will Addison is the obvious candidate to be drafted into Joe Schmidt's squad.
Addison was scheduled to start at full-back for Ulster in Saturday's pre-season friendly in Glasgow but a late change saw him dropping to the replacements.
Henshaw missed Ireland's two opening games at the 2015 World Cup because of a hamstring injury, although he played in his country's final three games in the tournament.