The former Liverpool striker suffered a thigh injury early in the Galati game that looks set to condemn the 31-year-old to another stint on the sidelines.
"He is having a scan done but he won't be available for Saturday," said manager Sir Alex Ferguson.
"Hopefully it is not too long but it is a killer because he has started the season very well. His performances in training have been terrific.
"We wouldn't have scored that goal if Michael had not made his run so it is a bit of a blow."