Kieran Marmion faces a race against time to be fit for Ireland's Six Nations title defence after Connacht confirmed he will undergo ankle surgery this week.
The Connacht scrum-half delivered a top-drawer performance in the absence of British and Irish Lions talisman Conor Murray as Ireland toppled New Zealand 16-9 in Dublin on Saturday.
The 26-year-old pulled off that showing despite carrying the ankle problem he had exacerbated in the 28-17 win over Argentina in Dublin on November 10.
"Kieran Marmion will undergo surgery this week on an ankle injury and will be out of action until February," Connacht confirmed in a statement.
Marmion limped out of the late stages of Ireland's win over the Pumas on November 10 but shook off that complaint in time to start against New Zealand on Saturday.
Ireland pulled off just their second-ever win over the All Blacks last weekend, and their first victory over New Zealand on home soil.
The Irish will now close out their autumn campaign by hosting the USA on Saturday, and must do without both Marmion and Munster star Murray, who is only just back in full training after long-term neck trouble.
Luke McGrath and John Cooney will now be in line to double up as Ireland's scrum-halves against the USA.
Ireland claimed their third-ever Grand Slam by beating England 24-15 at Twickenham in March, and will host Eddie Jones' men in the eagerly-anticipated rematch to start the 2019 Six Nations.
Sean O'Brien also faces three months on the sidelines after surgery to repair the broken arm he suffered against Argentina, leaving the Lions back-rower another Six Nations doubt.
Leinster have also confirmed Dan Leavy is dealing with a neck issue and faces a period of rehabilitation, after the flanker missed Saturday's New Zealand match with what Ireland had branded "general tightness".