The Brazilian was withdrawn 62 minutes into the 1-0 defeat to Sporting Lisbon on Sunday with a torn Achilles tendon as Castro suffered a defeat for the first time during his tenure.
Castro confirmed that his keeper's season appears to have come to a premature end.
"Helton's injury is serious. I think it is a complete rupture of ligaments. I'm no doctor but it is natural (to think) it should have finished his season," he told Sport TV .
Castro felt Helton's injury was a factor in their defeat to title-chasing Sporting as well as fatigue after their UEFA Europa League victory over Napoli on Thursday.
"The injury to Helton and the game we had on Thursday were factors that limited us a little," he added.
"I leave with the impression that the players, for all that they did, could have got a different result.
"Sporting got stronger but we managed to play our game. We had five clear chances. There were times when the team broke up a little or was not as joint."
