White will be a big loss against the tough-tackling Pacific Islanders

The 28-year-old flanker needs further tests to assess the damage but Hadden is anticipating bad news.

He said: "Jason is still to be seen by the specialist, but it appears that he may be out for some considerable time.

"I don't think we'll know for certain about the Six Nations until the specialist has had a look."

White will certainly miss Saturday's Test against the Pacific Islanders at Murrayfield and is not expected to face Australia on the following weekend.

Scotland team doctor James Robson said: "The scan has revealed damage to some of the structures within his knee.

"Jason will now be getting a specialist orthopaedic opinion tomorrow (Tuesday). Following that assessment, the prognosis will be clearer."

Centre Rob Dewey is also a doubt for Saturday's match after suffering a knee injury against Romania.

Glasgow backs Dan Parks and Andy Henderson and the forward trio of Ross Ford (Borders), Alastair Kellock (Glasgow) and Simon Taylor (Edinburgh) have been added to Scotland's squad.

Taylor has been out of action for six months due to reconstructive surgery on his knee.

Taylor may make a surprise comeback at the weekend

However, the 27-year-old managed to play for an hour of Edinburgh's Magners League game at Newport Gwent Dragons on Friday.

Hadden said: "I watched that game with interest on Friday night, and obviously we have monitored Simon's progress.

"He has trained with us all season, and his rehab has gone extremely well, so he is definitely an option we are considering for the weekend.

"Obviously, our medical team have been working with him for a long time, and are happy with where he is at the moment, but there is no way would take a gamble.

"We will be discussing what the next step for Simon is, in terms of what is best for him and what is best for everyone else."

The third member of last season's back-row, Allister Hogg, is also missing the Autumn Test series with a knee injury.