Manchester United have been preparing for their summer 2020 transfer window for a while.

Such is the operation at Old Trafford that the club hierarchy are only looking to bring in players in January that are on their summer transfer wish list that become available now - and for a good price.

This United side continue to be stung by mistakes in previous transfer windows. The biggest being their failure to replace players who have left. Hate him or love him Marouane Fellaini wasn't replaced, nor was Ander Herrera, a player who could and should have stayed at the club, and then it was Romelu Lukaku who left in the summer without a replacement coming in.

There might be outgoings in the January transfer window but none that should damage the squad. Nemanja Matic is keen to seek an exit but the midfielder would be no miss to Solskjaer's side. Paul Pogba, however, would be.

It was a somewhat baffling decision by the United manager not to start the France international for Newcastle's visit, especially after he suggested just that after the defeat at Watford, and the midfielder came on as a second half substitute for Scott McTominay.

"Paul (Pogba) it's too early to demand too much of him. He's worked really hard but then it's been a long lay off. We'll have to drip feed him in and gradually play him more and more," Solskjaer told MUTV before the game.

It had been a lengthy lay off for the 26-year-old, 12 weeks to be precise, and his importance to this United side has never been clearer. They've lacked creativity and composure in the middle and while Fred and McTominay have upped their games United look a better outfit with the World Cup winner in the starting XI.

You only needed to watch the last 20 minutes of the defeat against Watford to see how key he is and the impact he had was instant. At Vicarage Road he created three chances and against Newcastle on Boxing Day he tested Martin Dubravka twice with decent efforts and he was unlucky not to get on the scoresheet.

It's rare to find a player who has the ability to lift a team but that's what happened when he was introduced at the break. He strolls around with an air of calmness and confidence and provides goal threat and his presence, after coming off the bench, changed the way United controlled the game.

He's a welcome boost, especially in such a busy period, but Solskjaer and the hierarchy should already be looking at how they replace him.

Mino Raiola, Pogba's agent, poured cold water on speculation suggesting Pogba would leave the club in the January transfer window earlier this month.

"Paul wants to be successful and happy and wants to win prizes and he would love to do that with Manchester United," Raiola toldThe Telegraph. "Okay, there was big interest from Real Madrid and Manchester didn't let him go. We didn't make a fuss about it and he didn't make a fuss about it because I work in accordance with the player.

"You know other players can bring things to extreme but Paul doesn't like that. So you work differently. There was an interest from Real Madrid but they didn't let him go and I’m sorry, because I think that Paul being French then Zidane is an important factor in French football history."

A move away in January is highly unlikely. Why would the club sell their best player without having the time to replace him? But when the summer transfer window opens it's going to become harder to persuade him to stay.

Pogba's already flirted with a move to Real Madrid and he's out of contract in 2021. United can't afford to find themselves in a situation where they're going to have to let the midfielder go on a free transfer and the France international is not close to agreeing new terms.

His passing, movement and ability are unrivaled, as Old Trafford saw in the second half against Newcastle, and when the inevitable happens and he leaves the club United need to ensure his replacement is waiting in the wings. They cannot afford to be stung again.