Johnny Sexton is in a race to be fit for Ireland's Six Nations opener against Scotland on 1 February.

The 34-year-old is set to miss the remainder of Leinster's European Champions Cup Pool One campaign having injured his knee during last weekend's win over Northampton.

Leinster boss Leo Cullen did not give an exact timescale for Sexton's return.

"You would love if he was back (for Leinster), but it's probably more realistic for Ireland," Cullen said.

"He's seeing somebody in the UK and he'll go back there just to get an update in a couple of weeks.

"He's working hard to try and get himself back as quickly as possible, but he won't be a millions miles away hopefully."

Ireland play Scotland at the Aviva Stadium in their opening match of the tournament.

A scan on Monday revealed Sexton had suffered a significant medial tear early in the second half of Leinster's impressive 43-16 win at Franklin's Gardens.

The injury could present new Ireland head coach Andy Farrell with a selection headache at fly-half.

Sexton's World Cup understudies Joey Carbery and Jack Carty have both encountered difficulties since returning to their provinces.

Carbery is still nursing an ankle injury which restricted his playing time in Japan while Carty has found himself vying with Conor Fitzgerald for the number 10 jersey in Connacht.

JJ Hanrahan has started at fly-half for Munster in Carbery's absence while Leinster's Ross Byrne and Ulster's Billy Burns, uncapped at international level, will hope their impressive starts to the season will have done enough to warrant a place in Farrell's pre-Six Nations training camp.

Another decision facing the new head coach before the start of the tournament is that of the captaincy following Rory Best's retirement.

Sexton, a vice-captain alongside Peter O'Mahony for several seasons, skippered his country from the start for the first time against Russia in Japan.