Ireland flyhalf Joey Carbery has been ruled out for four to six weeks after sustaining an ankle injury in their opening Rugby World Cup warm-up, handing coach Joe Schmidt a selection dilemma just over five weeks before the tournament begins.

Carbery’s emergence as Johnny Sexton’s undoubted understudy has typified the depth Ireland has built up since the last World Cup when injuries to key players, including British and Irish Lions outhalf Sexton, derailed their chances.

Munster No.10 Carbery was the best player on the pitch for 50 minutes against Italy on Saturday before leaving in tears after his left leg got trapped under a ruck.

“Joey Carbery will rehab his ankle injury with the national squad and is expected to be available for selection in 4-6 weeks,” the Irish Rugby Football Union said in a statement.

Ireland’s other reserve halves – Connacht’s Jack Carty and Leinster’s Ross Byrne – have five caps between them, none of them as a starter.

Schmidt may therefore opt to bring a half-fit Carbery to Japan, hoping he recovers for their later pool games.

Ireland begin their Pool A campaign against Scotland on September 22 before facing hosts Japan six days later. Their final two group games are against lowly Russia and Samoa on October 3 and October 12.