Sunderland manager David Moyes has revealed Patrick van Aanholt did not feature against Tottenham due to the results of a cardiology test.
The Dutchman was initially named in the starting XI for Sunderland's 1-0 defeat, but was replaced by Jason Denayer shortly before kick-off.
Van Aanholt's late withdrawal led to confusion on Sunday, with Moyes' initial comments that he was left out for "medical reasons" failing to clarify the left-back's situation.
Moyes has now stated Van Aanholt's omission was down to "medical advice from cardiologists", with the results of a routine examination understood to have come in shortly before the match was due to start.
"He had the statutory checks with the cardiologists and the FA panel of cardiologists looked at it and advised us really late on Sunday that he should not play until further tests had taken place," Moyes told a news conference ahead of Sunderland's third-round EFL Cup meeting with Queens Park Rangers on Wednesday.
"We are just surprised that the FA panel didn't contact me or contact my [club] doctor until 3.55pm on Sunday afternoon. I have never in my career been put in a situation like that ever, so we had that to deal with.
"We just can't understand why the message would come to us at that time, 30 minutes before the kick-off.
"It was completely outside our control, but it was in our control because we knew we had to make the correct decision for the player and for the football club. We had to do that."
The Sunderland boss added that the 26-year-old "has undergone tests and will travel to London", although it is not sure yet whether Van Aanholt will play against QPR.
The former Chelsea player has made five appearances in all competitions this term, scoring once.