The opera singer underwent surgery last Thursday after he fell ill in the recording studio and an MRI scan showed he had a brain tumour.
Surgeons had hoped they could operate on the tumour through his nose, but Watson still has scar tissue from two golf-ball sized tumours he suffered a year ago, so they were forced to enter via his mouth.
Richard Thompson, the singer's manager, told The Times: "There was some concern because when he came out of theatre he was having trouble speaking, but he's speaking normally now.
"It is a worry, but we don't know how much of a worry. When he had the operation a year ago we didn't want to go through the mouth for obvious reasons, but there was no option this time.
"He's got stitches in his mouth and they'll need to heal. But he will be singing again."