The defender is a near ever-present in the Venezuela squad and featured for his country against Brazil during the last international break.
But having picked up a niggling injury at the KC Stadium on Saturday, Boro and the Venezuelan FA held discussions and decided it would be best practice for the player to stay on Teesside.
"He finished the Hull game with a little problem," said Karanka when asked about Amorebieta's omission from his national squad.
"He had a scan on Monday and we sent that to Venezuela - in the same way we did to Uruguay with Stuani and all other players.
"Our doctor was in touch with theirs and we decided to keep the player here because he wasn't available to play those games."