The Cameroonian champions reached last season's semi-finals before being eliminated on penalties by eventual winners Al-Ahly.
However, they were beaten in the first leg of their second qualifying round tie for this year's competition by their Algerian counterparts Setif on Sunday.
An El Hedi Belameiri strike in first-half injury time was enough for the hosts, who will take a slender advantage to Garoua for next weekend's second leg.
Four-time Champions League winners Mazembe, meanwhile, also face an uphill struggle to reach the group stages after their 2-1 loss at Sewe Sport.
Ivorian champions Sewe took the lead courtesy of striker Roger Assale nine minutes before the interval, and looked to have one foot in the competition proper when Christian Koffi Kouame doubled their advantage from the spot in the 85th minute.
However, Mbwana Samata pulled what could be a crucial away goal back with just a minute left to play, meaning the tie remains in the balance ahead of Friday's second leg.
South African champions Kaizer Chiefs' hopes of qualifying for the tournament proper appear all but over after they were beaten 3-0 at Vita Club, with Firmin Ndombe notching a hat-trick at Stade Tata Raphael to put his side firmly in control.
Elsewhere in Sunday's Champions League action, Congolese outfit Leopards de Dolisie were held 1-1 by Al Hilal Omdurman, while CS Sfaxien grabbed a 1-0 win at Horoya courtesy of Fakhreddine Ben Youssef's strike on the hour.
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