Reading manager Brian McDermott is waiting to discover the extent of a knee injury suffered by goalkeeper Adam Federici against Watford.
Federici was replaced six minutes from time on Saturday with Alex McCarthy coming on for the closing stages of the 1-1 draw at the Madejski Stadium.
The Australian is rated doubtful for Tuesday's Championship game against Millwall and McDermott is unsure of the severity of the injury.
"He has a knee problem," said McDermott. "We don't know exactly what it is yet.
"Alex [McCarthy] came in and did a good job for us so we will have to see what happens.
"We'll have a look at him in training."
Federici has been an ever present for Reading in the Championship this season and, if he is not fit to face Millwall, McCarthy will be in line to start his first game for the club.