Currently, Uruguay does lineup with a back 4. But i think a few years prior to the World Cup, they often played 3 CBs, seeing that Caceres is a versatile defender.
I haven’t seen Rennes play at all this past season, but they do have a good defensive record. Although, looking purely at stats, i remember seeing they never kept a clean sheet with Gnagnon on the pitch. 
But that match he played vs Atlético was so good, i could only hope he recaptured some form. I wouldn’t mind him as 1 of our 4 CBs next season.