The 21-year-old Munster centre picked up a shoulder injury in the first-half of the 37-25 triumph over the Royal XV.
It wasn't until Lions doctor, James Robson, revealed the extent of the injury that anyone knew the debutant had been hurt. His right shoulder was damaged at the clavicle region when home prop Albertus Buckle landed heavily on him.
The injury was very sore after the game and he will not be able to train for two or three days.
"Keith had one of those games that we've all had - we've all got the T-Shirt," said Lions backs coach Rob Howley.
"He will be disappointed. He is young, he made mistakes, but I'm sure he will bounce back.
"Every player on this Lions tour has gone out and found that, for whatever reason, the ball hasn't bounced their way. It happened to Keith yesterday and we are not hiding away from that fact.
"Wearing the Lions badge can sometimes make you feel 6ft 8in tall, but then you go out and drop the first ball and all of a sudden you are trying to get your game back on track. You could see Keith's frustration when he kicked that ball out.
"He will be a 10 times better player for that experience and I'm sure he will bounce back and show why he was picked for the tour in the first place. The player himself and his team mates will pull him through."
Brian O'Driscoll was one of the first players to go up to Earls after the game and he, along with his centre partner on the day, Jamie Roberts, was full of praise for his battling performance.
"The only way you learn is by playing. I was pleased to be playing alongside Keith to form such a young centre pairing," said Roberts.
"I'm sure he wouldn't have been pleased with some of the dropped balls, but that's the way it goes some days. I'm sure he will get the chance to put the jersey on again and do it justice, because he is a hell of a player."