The 25-year old Spanish centre-back ruptured ankle ligaments in the 4-1 win over QPR on Saturday and is now in doubt for the Capital One Cup final with Bradford City on February 24 - a game Rangel hopes he can recover in time for.
"We spoke to him at half-time and he didn't look very happy," Rangel told the South Wales Evening Post.
"He's been a very important player for us. He's been a rock at the back with Ashley Williams and he will be a big miss.
"We know we have other players who can fill in and do well. But I am very sad for him because he's had an amazing season. Hopefully this won't be too serious.
"I am sure if it's not anything serious like a broken bone or ligaments he's going to try to rush back for Wembley.
"It's one of the finals you've been dreaming of all your life and he knows that. He wants to win a trophy because he's never won one."