The 20-year-old, who had been in line to make his first-team debut for the club at Aston Villa on Saturday, damaged his right knee is training last week, but manager Alan Pardew is hopeful he could return to training within days.
"He has gone to London today (Monday) and will have his knee drained of a little bit of fluid that's on it, and then we are hoping that he will train," said Pardew.
"If that doesn't work, probably at some stage, we will have to look at some keyhole surgery in there to make sure that this fluid doesn't appear again."
Santon's absence is a blow for Pardew, who signed him from Inter Milan last month to replace Jose Enrique following the Spaniard's move to Liverpool.
Santon undergoes knee treatment
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