Phillips, 22, has missed Albion's last three pre-season friendlies with what has now been diagnosed as a hairline fracture in the fifth metatarsal.
"I think Jimmy had an X-ray and they just felt as though there's whatever's before a stress fracture - just a very, very small hairline possibility of a fracture," said Burton boss Gary Rowett. "I think we're gonna have to be cagey with Jimmy and it might be between four to six weeks unfortunately.
"(Physio) James (Rowland) has been precautionary with him anyway, so again, it's just a big blow."
Rowett is facing an increasingly congested treatment room with strikers Calvin Zola (hip) and Justin Richards (hamstring), left-back Aaron Webster (hamstring) and midfielder Jack Dyer (knee) all nursing problems.
Saturday's 2-1 home victory over Walsall also came at a cost, with midfielder Jacques Maghoma limping off early in the second half with an ankle injury, while right-back Andy Corbett did not emerge for the second half against the Saddlers as a precaution after complaining of stiffness in his groin.
