Tahu, who played the opening three rounds of the Super 14 for the New South Wales side, was forced to pull out of the local derby against the Brumbies a fortnight ago after straining his hamstring in training.
The injury has not recovered as well as originally hoped and the former Parramatta Eel could be facing an extended spell on the sidelines in order to get his hamstring right for the final rounds of the competition.
Although disappointed to be missing the clash against the unbeaten Crusaders, Tahu said it was more important to getÂ the long-term benefits from the rehabilitation period.
"It's just not right at the moment," said Tahu.
"The medical staff don't want to push it too much so we just need to get it right first and make sure it's 100%.
"I'm going to be here for four years - I'm not going anywhere - so I think if I get can everything right, and not have this problem of always having hamstring problems, maybe next year I can get back for a full season instead of having hiccups along the way."
Coach Ewen McKenzie added that while it was frustrating to lose Tahu, the setback provides an opportunity for the club's medical staff to finally conquer the player's troublesome hamstring problems.
"There's a bit of a loop happening in terms of the injury site, and the way it's occurring," said McKenzie.
"What we don't want to do is keep having the same cycle. To break that cycle will take a little bit longer and we'll get it right once and for all."
McKenzie said the Waratahs medical staff would look at making biomechanical changes to Tahu's training programme.
"We've done this in the past," he said. "We did it with Rocky Elsom four years ago.
"He was having a similar problem with repeated issues in the same area, so we took a bit of time and got it right.
"At the moment Timana's going pretty well and we're happy with his improvement, but we want him playing at 100% and I think at the moment we're getting 85-90%."