Simplify the following text for a Grade 7 native English-speaking person.
Original: This would be the start of a chronic knee problem - injuries to his left knee caused him to miss significant action 6 times over the next four seasons , and required three separate surgeries .
Simplified: This would be the start of a chronic knee problem . Injuries to his left knee caused him to miss significant action six times over the next four seasons , and required three separate surgeries .