The Republic of Ireland international was carried off on a stretcher in the first half of Tuesday's match against Kazakhstan and less than 24 hours later the club announced he would require an operation on his knee.
However, Martinez insists the 25-year-old's season is not over and he hopes the midfielder, who has been troubled by a number of injuries over the last 12 months, can return stronger.
"It was a real disappointment on Tuesday night," said the Spaniard. "I was watching the game and straight away you could see it was an innocuous incident and that is normally when you pick up really bad injuries.
"I was really fearful of expecting the worst but with the scans on Wednesday we are very positive now. It is an ACL but the whole structure of the knee is perfectly fine and it is going to be straightforward surgery.
"I'm looking forward to seeing him back hopefully this season and that in a way was good, positive news taking into consideration what it could have been.
"The ACL is ruptured but his knee reacted really well and nothing else was damaged - in those sort of actions you normally get more than one structure damaged and some of those are very difficult to repair.
On the impact Gibson's loss will have on the squad Martinez added: "You don't want to lose players and in pre-season I felt Darron was magnificent.
"He brings something we haven't got in the squad; his understanding on the ball and the way he was starting that partnership with James McCarthy was going to be important.
"But it is no good to look back, we are just looking forward and we want to see Darron fully fit as quickly as possible."