The 53-year-old says the absence of the Ballon d'Or winner will not aid his team in their task of beating Barcelona on their own turf
The Argentina international did not re-appear for the second half of Tuesday's 2-2 draw in Paris and it was later revealed the 25-year-old had sustained a hamstring injury.
However, having played no part in Barcelona's victory over Mallorca at the weekend, it remains to be seen if coach Tito Vilanova will include the star in his plans for the visit of the French giants on Wednesday.
And speaking to RMC Sport on Monday, the former Chelsea manager says Messi's potential exclusion does not make the task of progressing to the next round any less unenviable for his team.
"We have to defend, but our goal is not only to defend, we must also show courage," he said.
"With or without Messi, it is the same for us. To reach the semi-finals, we still have to win."