The blow to the liver sent shockwaves through his body, a weaker man would have siezed and dropped instantly. His body lurched forward in response to the blow, his face showed a definite sign of surprise upon it. Namely with the spray of saliva being shot from his mouth and his mouthguard nearly being knocked clean out by the force of his exhale. In the moment between lurching forward and the oncoming uppercut, he needed to react.

But how? He was pretty much set for that incoming blow, except for one thing. Just because he was right handed didn't mean that he was defenseless. Three movements worked in a flash, and it was purely instinctive. 

Fergus threw his head backwards and put his weight down on his right leg and jerked his waist off to the right. The uppercut swung up and missed his jaw, the wind grazed his ear and threw a tuft of red hair upwards. But that wasn't the whole motion, see Fergus is a reactive fighter, and his reactions are generally what wins his fights. 

Remember how he brought his left arm back? Well his guard is that way for a reason, an uppercut is a guarateed arm lock that his opponent throws themselves into. It was even better since The Undertaker was fighting south paw, and since this was a left handed uppercut, he was able to hook his hand right into the movement. He pulled his own left arm into his body, forcing the Undertaker's hand in with it. 

The last movement was a full left leg sweep, since his hip was already twisting from the reaction, his leg moving in a wide crescent was a natural flow. Aimed straight for the undertaker's legs and with his dominant hand locked straight in check, this would quickly be taken to the floor with Fergus in the advantageous position. 

If executed perfectly, Fergus would have the Undertaker facedown on the ground with his left arm in a lock. He'd be able to perform a full mount and something resembling a rear wrist lock.