Basically being left doing the midfield job alone. Fellaini is limited and used mostly to support Lukaku in the box which means he would take long to come back to support defense. Lingard is not central midfielder. He can put in a shift but not effective to win the ball back nor ignite an attack deep.
Unfortunately, Pogba sub was total disaster. Lingard was returned to advanced position, but Pogba didn't help. Basically left Matic to deal with all runners at once while also burdened to help getting the ball out from the back. Our defensive line was sloppy at it to begin with and with less help it turned into disaster. The first goal was what it was: poor sloppy build up, easy to read, and broken by opposition. Not enough number to stop the break and the chance was created very simple.