Mitchell said his slow recovery from ankle and hamstring injuries sustained during the past year had been ''the toughest thing I've had to go through in my career'' and was still a work in progress.
Although the Waratahs are yet to announce the team for the grudge match against the Reds, which opens their Super Rugby season, the Wallabies wing ruled himself out of contention at the club's season launch yesterday, saying his ankle just did not feel right.
''It's really more long term for me,'' the 27-year-old Mitchell said. ''With the area [the injury is in] and the issues I've had with it, there is a chance it could impact me long term and I can't let my excitement come over the top and make the decision for me.
''If I thought I could, I definitely would be playing but I guess I know my body pretty well and I'm at the point now where I've resigned myself to the fact I won't be playing in that game.''
The admission is a blow for the Waratahs, who lost their other top-line winger, Lachie Turner, to a hamstring injury in the trial match against Samoa last week.
But Mitchell was never more than a possible to start the season after surgery to remove a bone spur from his ankle in November revealed a four-centimetre tear in the tendon as well. A 12-week recovery was ordered.
The year has been horrific for Mitchell, who dislocated his ankle in April but recovered in time to score two tries for the Australian Barbarians against Canada in August and make Australia's Rugby World Cup squad. He then tore his hamstring in the Wallabies's final pool match, against Russia, in October.
''It's tough, I grew up playing a team sport, I'm employed to play a team sport and that's what I love and for the most part of the last nine months I've been training as an individual, and I guess motivation's a bit of an issue at times when you're getting up and you don't have the other boys to feed off,'' Mitchell said.
He has spoken to Turner since the announcement his team-mate would likely miss most of the Super Rugby season and offered an ear - but hopefully not a seat on his couch.
''[Lachie's] going to face some pretty dark times when
you're sort of sitting on the lounge and it seems like hours on end and
it often is … Hopefully I won't be sitting next to him but it may turn
out that way.''