Leinster have confirmed that Johnny Sexton did fail a head-injury assessment (HIA) during the province's victory over Exeter last Saturday and is undergoing return to play protocols.

The outhalf took a knock to the head and was forced off after just two minutes at the Aviva Stadium but Ireland head coach Joe Schmidt said at the RTÉ Sport Awards that night that he believed Sexton had passed the assessment.

"[Sexton] passed his HIA," said Schmidt. "He’s okay, I spoke to him this evening. He’s fine, he’s likely to be back playing..."

However, the province today clarified that Sexton had not passed the examination in full.

"He passed elements of the HIA but not all of it so therefore he wasn't deemed fit to come back on," senior coach Stuart Lancaster told RTÉ Sport.

"Obviously the player comes first and that's the right decision.

"I spoke to him after the game and he seemed well. He'll go through that process (return to play) this week."

Sexton may now be a doubt for the St Stephen's Day Pro14 showdown with Munster, depending on how doctors assess his condition this week.

The Dubliner suffered four concussions in 2014 and was forced to take a 12-week break that ruled him out Ireland's opening game of the 2015 Six Nations.

A perceived tendency to tackle high has been blamed for many of those knocks but Lancaster doesn't think that was the cause of Saturday's injury.

"We work on tackle technique all the time," said the Englishman. We want to protect players so they can play the full 80 minutes."

"That particular incident, we got tight defensively and Johnny ended up having to come in. So it was more a defensive system rather than a Johnny tackle technique.

"We needed to be wider at that breakdown and then probably Johnny isn't even involved in that. Sometimes you have to make split-second decisions and late adjustments against guys who are running hard at the line and you're trying to stop them.

"How do you protect a player in a game of rugby? It's a combative game and there is a risk you're going to get bumps and bruises along the way.

"But quite rightly, if someone gets a knock it needs to be assessed.

Sean O'Brien will definitely miss the derby while hooker Sean Cronin broke two teeth during the Exeter game but is expected to be available for the trip to Thomond Park.

"General knocks, bumps and bruises," Lancaster explained of O'Brien.

"He's had two tough games in two weeks and he's been at the forefront of everything that we've done really.

"He won't be available this week but we don't consider it serious or long-term."