Patchell was helped off the field after half-an-hour of the Blues' 21-13 PRO12 triumph on New Year's Day and scans revealed he will need surgery on his damaged left knee.
Gareth Davies, who scored the match winning try for the Blues against Toulon in Round 2 in October, is the likely replacement for Saturday's Round 5 fixture in Nice. With Toulon two points ahead of the Blues in Pool 2, whichever team wins at the weekend will be favourites to take a quarter-final spot.
"We've got confidence in Gareth and he has done a fantastic job for us this season. He came on against Toulon in October and won us the game," said Blues boss Phil Davies.
"We know we have got a big job ahead of us, but Gareth has got the ability. It's about getting him to show what he can do and lead the team around the pitch."
Toulon currently top the pool with 15 points from four games having picked up maximum points in their two home ties so far, but the Blues have happy memories of a previous fixture against Jonny Wilkinson and co in France when they claimed Amlin Challenge Cup glory in Marseilles.
The Blues ran out surprise 28-21 winners three years ago to become the first Welsh side to win a European trophy and a repeat of that success would mean that victory over Exeter in Cardiff in Round 6 would secure a quarter-final spot for the second time in three seasons.