Toral had been in good form recently, but damaged his knee in the 3-0 win at Fulham last weekend before scans revealed the extent of the damage.
“Jon will be out for around three months and he had an operation in Barcelona on Friday night,” said McCann.
“It’s a big, big blow for Jon because he was coming into form but it is what it is. It was a twist in the game at Fulham that he probably didn’t know he’d done. Then training on the Monday his knee buckled underneath him.
“We’re all devastated for him, but hopefully Jon can be strong and come back from this.”