Ospreys flanker Sam Underhill will miss the first half of the season after suffering a shoulder injury that requires surgery.
His loss is a huge blow to the region who have already lost Wales U20s star Owen Watkin for most of the next campaign with a knee problem.
Underhill was arguably the most eye-catching breakthrough performer in Wales last season, although he qualifies for England and has ambitions to wear the red rose.
A student at Swansea University, the 20-year-old took the Guinness Pro12 by storm, in an otherwise underwhelming season for the Ospreys.
Bigger things were expected of him from next month – especially with fitness concerns still surrounding flanker Justin Tipuric – but an injury picked up in training will now see Underhill have an operation later this week and he is expected to be out until the end of the year.
Ospreys medical performance manager Chris Towers said: “Sam suffered a high-grade AC joint injury in training last week, since when he has had scans and we’ve consulted with a specialist.
“The agreed course of action is for him to undergo surgery to reconstruct the damaged ligaments. That will take place at the end of the week before he commences a programme of rehabilitation.
“The initial prognosis is that he can expect to be sidelined for a period of around four months.”
Underhill’s impact last season had been noted by England coach Eddie Jones who was keen to explore the possibility of capping the back rower.
That would have prevented any chance of Underhill switching allegiance to Wales and trying to qualify through three years residency.
But Jones was told that Underhill is out of reach as he currently plays outside of the Aviva Premiership and that his circumstances do not qualify as “exceptional.”
That prompted speculation that Underhill may try to seek a move to Bath, although he still has two years left on his current Ospreys contract.
