New Zealand captain Kieran Read could be sidelined for up to six weeks after having surgery on a fractured thumb.
The number eight sustained the injury in Canterbury Crusaders' 48-21 victory over South Africa's Cheetahs in Super Rugby on Saturday.
The 31-year-old, who has won 97 caps, is expected to lead the All Blacks in their three-Test series against the British and Irish Lions.
The tour begins on 3 June with the first Test in Auckland on 24 June.
Read left the field just before half-time in Bloemfontein on Saturday and later had surgery on a fracture at the base of his right thumb.
"It is extremely disappointing for Kieran and the All Blacks," said Crusaders assistant coach Jason Ryan.
"The guy has worked so hard off the field to get his hand right from the surgery he had during the off-season and has been world-class since returning to the field.
"Obviously it's disappointing and will be concerning for the All Blacks."
Read also missed the first six rounds of the Super Rugby competition and only returned to action on 14 April following his wrist surgery.
Flanker Jerome Kaino, a two-time World Cup winner, is also expected to be out for four to six weeks after surgery on a meniscus tear, but is likely to be fit for the Lions series.
The Lions, who will be captained by Sam Warburton, start their tour against a New Zealand Barbarians side in Whangarei on 3 June.
BBC Radio 5 live rugby reporter Chris Jones
"Considering all the cards are firmly stacked in New Zealand's favour, any slight setback to the All Blacks' preparation will provide some much-needed encouragement to Lions supporters.
"Kaino has long been a class operator on the blindside, but at the age of 34 is perhaps not guaranteed his place in the side; Read on the other hand is one of the All Blacks' talismen, an ever-present at number eight since his debut in 2008, and if he doesn't make the first Test, he will be sorely missed."