Marseille, who visit mid-table Valenciennes on Sunday, finished a dismal 10th last May but are enjoying their best league start in more than 50 years after taking a maximum 18 points from six games under new coach Elie Baup.
The leaders are four ahead of second-placed Olympique Lyon but Morel reckoned the 2010 French champions, who were not expected to challenge for the title this term, have to keep working harder than ever.
"We are fully aware that we cannot afford to ease off, otherwise we will quickly be struggling again," Morel told a news conference.
"The ghosts have not gone away yet. Everything will be ruined if we stop making an effort."
Despite his caution, Morel suggested Marseille are already eyeing their next week's home clash with arch-rivals and title favourites Paris Saint-Germain.
"In order to prepare for the PSG game as well as we can, we know we cannot bungle at Valenciennes," he said.
The 2011 champions Lille visit 18th-placed Stade Rennes on Friday aiming to clinch their first league win since the season's opener.
Lille bounced back last weekend after a shock 3-1 home loss to Belarus' BATE Borisov in the Champions League by producing a convincing display in a 1-1 draw with Lyon.
"Rennes have one of the best attacking lineups in the league, as I have always said. They will be able to score at any time," he said.
