The Whites winger flew into a first half tackle that left Kovačić writhing in agony on the Elland Road turf and, eventually, unable to carry on. Referee Anthony Taylor gave himself a second to consider his options and then produced a red card for James, making it two dismissals in two games for Leeds.
Chelsea, comfortable from the start, went on to win 3-0 and consigned Jesse Marsch's Leeds to a third straight defeat. They now must hope they can pick up a sufficient points haul from their last two games against Brighton and Brentford to overtake Burnley, who are level on points but with a much better goal difference and a game in hand.
Speaking after the Blues' victory, Tuchel expressed his fear that Kovačić will miss this weekend's FA Cup final against Liverpool.
"I tell you not as a coach or an expert, it's very unlikely," he said.
"He never goes out, he played in the Carabao Cup final and in the FA Cup semi-final with the same injury. He tried to play another 15 minutes which is nearly impossible. He has a swollen ankle and is in a lot of pain. It's very unlikely he can play in the cup final, it's very unfortunate for us. If he misses it I'm very disappointed, he was a clear starter.
"If you look at the quality with him and Jorginho in midfield it was a very strong start, 11 v 11. He's super, super hungry and determined and excited to play these matches. We will see, we need to wait, maybe for a miracle and we can make it possible."
As for James' tackle, Tuchel said he only caught a glimpse of it on a screen in the Chelsea dugout but revealed his distaste for footage that shows his players being injured.
"I did not see the challenge live, I was focused on something else," he said.
"I saw it from a distance on the iPad. I'm not good at watching these things, it hurts me physically to watch it. Everyone told me it's a clear red card."
Tuchel was delighted with what he saw from his team at Elland Road, however as they snapped a three-game winless streak.
"I think we played very well from the very first minute," he said.
"I was very happy with the level of determination, we were strong in the challenges, brave in the defending. We were very strong 11 v 11 and later the focus shifts a little bit with one man more, you get easier ball possession. The level of focus needs to stay high, you need to be disciplined to avoid counter attacks because Leeds will never stop running and fighting. We understood the game better and better one man up in the second half and we did not give in. That was the most important thing. It was a good performance for a deserved win."