The former French champions were held to a goalless draw by Ajaccio at home on Friday.
"I've said for a while now that we have to keep working and pick up points, but it'll come down to the last six games," he told reporters.
"It'll be tough for everyone, and the qualities that we've shown up till now will come in useful in our last nine matches. It's not the end of the league this weekend.
"The league is tough as the teams near the bottom of the table can take points from those near the top, we've seen that with Paris Saint-Germain too. It's difficult for everyone."
Marseille are currently six points behind PSG in third place in the league table.