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.
Marseille are six points ahead of third-placed PSG, who are unbeaten and will seek a fourth consecutive victory at home to modest Sochaux on Saturday.
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.
They will still be without injured forward Salomon Kalou and playmaker Marvin Martin for the trip to the European hopefuls.
Rennes have also suffered a disappointing start but Lille coach Rudi Garcia, whose team have not recorded a clean sheet so far in Ligue 1, fears their fire power.
"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.
On Sunday, unbeaten second-placed Lyon host a Girondins Bordeaux side who have not lost a league game since early April but lie sixth after four consecutive draws.
Defending champions Montpellier, 16th on five points, travel to 19th-placed Nancy on Saturday before their Champions League game at Schalke 04 next Wednesday.
