Phillips sustained a thigh injury moments from the end of Blues' 1-0 defeat against the Spaniards at St Andrews on Saturday afternoon.
"He said it felt like a bullet in his leg, his thigh muscle exploded," McLeish told Sky Sports News.
"The early signs are not good, it looks as if he could be out for a while with that one.
"Hopefully the diagnosis over the next 24 hours proves different but he'll be getting scanned tomorrow.
"It doesn't look good for the wee man and more importantly for the club as a whole."