Despite being named in the starting line-up by Declan Kidney yesterday, O'Brien has been unable to train this week as he receives treatment for the infection.

Ireland Team Manger Mick Kearney said: "We knew Sean was carrying this infection in his foot, but unfortunately it has not cleared as quickly as we expected.

"While it has not healed enough for him to play this Saturday, we expect him to be fine for next weekend. With Sean out, we have decided to bring Peter onto the starting side and Shane Jennings into the squad."

The attrition rate from Ireland's 17-17 draw with France on Sunday has been high with O'Brien the third player to be made unavailable for Scotland.

On Monday captain Paul O’Connell and scrum-half Conor Murray were ruled out of the remainder of the Six Nations because of knee injuries.

Ireland’s schedule of four games in four weeks – a product of the weather-enforced postponement of their match against France – is stretching their playing resources.

O’Mahony is highly-rated and is a more natural openside than O’Brien, but the 22-year-old has only won two caps as a replacement against Italy and France in this championship.

The more experienced O’Brien came to the fore last season when his explosive ball-carrying made him a central figure for Ireland and Leinster.

The 25-year-old was switched from blindside to openside flanker during the World Cup and continued to bulldoze a path through opposition defences.

While he has not had the same impact in this Six Nations, he is remains a key component of Ireland’s vaunted back row.