Seán O’Brien has been ruled out of Leinster’s Guinness Pro12 semi-final against Scarlets at the RDS with a hamstring injury, Leo Cullen has confirmed.

The Lions flanker had been named among the substitutes for tonight’s clash (KO 7.45pm), but the Leinster head coach has confirmed that O’Brien is suffering from a tightness in his calf and a decision has been taken not to risk the injury further.

Ross Byrne comes into the bench in his place as the province had previously named just two backs on the replacements bench.

Cullen had recalled a number of his frontline players for the visit of Scarlets.

There are three changes to the starting 15 that faced Ulster two weeks ago, as Lions tourists Jonathan Sexton and Robbie Henshaw return to the side and there's a big vote of confidence in Ross Molony as he comes in at second row with Devin Toner dropping to the bench.

Joey Carbery switches to full-back, with skipper Isa Nacewa returning to the left wing.

Cullen has plenty of options at his disposal within his matchday squad, with Cian Healy, Zane Kirchner and Byrne joining Toner among the substitutes.

Lions hooker Ken Owens has been ruled out for the Scarlets due to an ankle injury.

Scotland flanker John Barclay will captain the visitors in the Welshman's absence. The Lions leave for their 10-game New Zealand tour on 29 May.

It is understood that Owens was injured during training with the Scarlets earlier this week and has already been assessed by the Lions.

The Lions, currently in Wales for a pre-tour training camp, are set to reassess him, probably ahead of next week's training get-together in Ireland.

Leinster: J Carbery; A Byrne, G Ringrose, R Henshaw, I Nacewa (capt); J Sexton, L McGrath; J McGrath, J Tracy, T Furlong, R Molony, H Triggs, R Ruddock, J van der Flier, J Conan.

Replacements: R Strauss, C Healy, M Bent, D Toner, D Leavy, J Gibson-Park, R Byrne, Z Kirchner.

Live coverage of the second semi-final between Munster v Ospreys  (6.15pm) and Northampton v Connacht in the Champions Cup play-off (4pm) on Radio 1's Saturday Sport.