#3 United do not have a ready replacement
In Solskjaer's midfield, Paul Pogba is the creative spark and Nemanja Matic is the midfield anchor. While the Frenchman makes up the glorious balls and is responsible for the goal, the Serbian is the defensive screen. Ander Herrera's position, as such, needs a varied range of skill.
Herrera can tackle, he can intercept. He can also spot a pass and make runs from deep. He even has the responsibility to drop down and help Matic cope with opposition pressure whenever required. He is a vital player for his team, and United, unfortunately, have no ready replacements in their squad.
The Spaniard averages the most tackles and interceptions per game among all midfielders in the United squad. He has also scored 3 goals and picked up 3 assists so far this season. None of his replacements in the squad - Fred, McTominay, and Pereira - can match up to the Spaniard.
As such United need to indulge in the transfer market to replace the Spaniard - a costly proposition with a lot of risks. Considering the poor outcome of the Fred transfer, United should be hesitant to make another mistake.