#2 Ander Herrera
Perhaps the biggest setback on Sunday would be the injury picked up by Ander Herrera in the first 20 minutes. The Spaniard pulled his hamstring and was forced to exit the game pretty early. His absence will be a deafening blow to United's current form, because, he is arguably United's most important player right now.
Herrera brings an infectious energy to this team that charges up the players. He is the unsung hero on the pitch, the dark night who rarely takes the credit but is integral to everything good coming out of Solskjaer's team. He is a leader on the ground and his intelligence makes him an invaluable asset to United.
The Spaniard is crucial in breaking down the opposition play and pops up everywhere on the pitch. He takes responsibility and is very clever with his assessment of the game. And when he has the ball, he uses his technical abilities and fantastic passing range to unlock defenses. He is the engine room in this United team that brings the best out of Pogba and Matic and together, the three of them has been at the heart of all the recent success.
As such, Herrera's injury will hurt Solskjaer the most. There is no player in the United squad who can replicate all the qualities of the Spaniard and his absence will give his Norwegian manager a lot of headache.