Learning what actually works:
The role of defender in football can be very important. [a general statement]
The role of the defender in that game was crucial. [that specific defender]
This does not involve countability or not. It involves specificity and generality.
- The role of defenders in football can be very important. has the same meaning as 1)
The role of mother is a hard one. [a general idea]
The role of mothers in our school is complicated. [general idea]
The role of the mother in that play is quite interesting. [specific]
Although there can always be exceptions, the structure of the examples above are very common. And, it does not matter about countability.
What matters is generality and specificity in an actual sentence.
The singular has been called a bare noun. Fine, but that does not explain the difference in meanings.