If you are told to start "supplementing" for any reason, start pumping! Pumped/expressed milk is the best supplement, followed by donor milk, and then by formula.
So many parents are led to believe they must start formula when asked to supplement. If babies start having formula and the parent is not pumping, that is when milk supply is at risk. Regular removal of milk (by an efficient baby ideally, or by a pump) is critical to build/protect supply, especially in cases where baby is struggling at the breast. As an IBCLC, it is so much easier to help get a struggling baby to breast with a robust milk supply already in place!
Here's a great article on "Hands On Pumping"
https://med.stanford.edu/newborns/professional-education/breastfeeding/maximizing-milk-production.html