Had Ivory Coast referee Noumandiez Doue seen the incident there is little doubt that Sakho would have been off, and it may be that he faces retrospective action from FIFA.
This was a tightly-fought affair from the start, with France shading the first half and Ecuador content to play on the break.
There were seven Premier League players in France's starting line-up, including first starts for Bacary Sagna and Laurent Koscielny, plus Southampton's Morgan Schneiderlin.
There were a series of half-chances for France - Antoine Griezmann was a yard wide with a low strike, then Moussa Sissoko had a swing with a left-footed volley which was too easy for Ecuador goalkeeper Alexander Dominguez.
The best chance came when Digne, Paris St Germain's impressive 20-year-old left-back who was also making his World Cup debut, won a free-kick. Griezmann floated it over for Paul Pogba to thump a header goalwards but Dominguez was able to tip it over.
After Ecuador's Christian Noboa was forced to don a hair-net to cover up a bloody wound incurred from a clash of heads with Blaise Matuidi, the South Americans began to improve.
Michael Arroyo made a surging run and Enner Valencia was waiting to pounce but the midfielder tried to cross with the outside of his foot and it was too close to Hugo Lloris.
Enner Valencia, easily Ecuador's player of the tournament, did win a good header but once again Lloris was equal to it.
Dominguez then denied Griezmann but he knew little about it. Sagna's cross was met by Griezmann's powerful header which bobbled off the keeper's hands and on to the post before being cleared.
Then came Valencia's lunge on Digne, raking his studs down the full-back's leg and had it not been for a shinpad the outcome could have been much worse. France coach Didier Deschamps looked concerned on the touchline but after treatment Digne was able to continue.
Ecuador nearly caught France napping with a terrific break by Enner Valencia and Noboa should have scored but his first touch was horrendous and he sliced his shot well wide.
Matuidi was next to threaten after a nice touch from Karim Benzema but Dominguez saved well again, before Pogba was horribly wasteful from a free header at the far-post.
Benzema failed to trouble Dominguez from 20 yards, then after a quick break Michael Arroyo shot wildly wide for Ecuador.
Ecuador continued to threaten on the counter-attack, and Alex Ibarra had a powerful strike beaten away by Lloris, then there were more heroics from man-of-the-match Dominguez to keep out Benzema, Loic Remy and Olivier Giroud.
It may have been a comedown for France after their high-scoring exploits against Switzerland and Honduras but Deschamps will not complain - Les Bleus are unbeaten and marching into the last 16.