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The first five-eighth appears touch and go for the Highlanders' first Super 15 match against the Chiefs in Hamilton next month.
One player who is definitely out is outside back Siale Piutau, who has an ankle in plaster and will miss the first month of the season at least.
Highlanders coach Jamie Joseph admits injuries to players aree not ideal as the side spends this week in Wanaka training, and mingling with the locals.
Slade has had a nightmare past 12 months on the injury front.
He broke his jaw in the first pre-season game for the Highlanders nearly a year ago and then broke it against the Rebels, shortly after getting back on the field.
He was selected for the All Blacks for the World Cup but injured his groin in the quarterfinal against Argentina and was forced out of the tournament.
He then picked up an infection from his groin operation, which further delayed his recovery.
Joseph said a small hernia had been diagnosed on Slade, which was linked to his groin injury, and it would be operated on today.
If the operation was a success Slade could be available for the final pre-season game against the Blues on February 17, Joseph said.
With his wretched run of injury problems over the past year, no-one is boldly predicting his return to the field.
Piutau has had an operation on a tendon in the ankle, putting him in plaster for six weeks.
Joseph said Piutau was probably looking at a best-case scenario of being back on the paddock at the end of March.
"It is a shame for Siale as he had been having a very good pre-season and had got into really good shape. That is coming off the back of an excellent first season for us and a good World Cup," Joseph said.
"A lot of guys still have injuries and are not fully participating in training. But that is the way things happen and it is better we be careful with them now and get them exactly right for the whole season."
With Jarrad Hoeata out for the first six weeks of the season after ripping his bicep muscle off his chest last month, the Highlanders have called in former lock Hoani MacDonald to their wider training squad.
Macdonald left the franchise after the 2008 season to play in Wales and then played for the Rebels last year.
He was off contract with the Rebels after playing in their inaugural season and has signed with Southland for the upcoming ITM Cup.
Just three locks had been selected for the Highlanders squad and, with Hoeata out for a lengthy period, Macdonald was a natural fit to come into the wider squad.
Midfield back Shaun Treeby is expected to increase his training load this week in Wanaka as he recovers from an operation to his knee.
Captain and prop Jamie Mackintosh and lock Josh Bekhuis are still recovering from off-season operations and are still a while away from taking a full part in training.
But Joseph was again hopeful of getting them some playing time in the pre-season games.
The first pre-season clash is only 10 days away with the Highlanders taking on the Crusaders in Greymouth on February 3.
They then face the Chiefs in Queenstown on February 10 before the final game against the Blues in Auckland.