Said had been on for less than 20 minutes when he neatly turned in a cross from right back Jonathan Clauss in the 88th minute on Saturday.
The win moved northern side Lens up to sixth place while Britanny-based Rennes was fourth before Sunday's games.
Both clubs lost their last two league games and it was a cautious first half, where Lens had more possession but failed to create clear chances.
Saturday's other game between defending champion Lille and Lorient was postponed after a dozen Lorient players tested positive for COVID-19.
Runaway league leader Paris Saint-Germain will be without Lionel Messi and several other players who had the coronavirus for Sunday's trip to Lyon.
Messi returned to individual training soon after testing negative for the virus on Thursday. But winger Angel Di Maria, midfielder Julian Draxler, central defender Danilo Pereira, left back Layvin Kurzawa and goalkeeper Gianluigi Donnarumma remain in isolation.