What does he do? Hide away from the ball, giving the Cb's no passing avenue. You can't be on the field as a midfielder, especially one with more creative responsibility, and allow your defense to struggle to progress the ball up the pitch - that's hiding.
Immediately Erickson came on. Watch how he slots into Martinez's space allowing the latter to push up the LWB position and giving us a passing avenue that immediately dissolved Southampton's press. Once we broke out from the back, he pushed back into midfield to help in possession. Exactly how we came about Amad's goal.
The boy should sit and learn more.
Unfortunately, Erickson is not 10 years younger.