Morgan Schneiderlin's transfer from Manchester United to Everton has been the biggest of the January window so far.

The defensive midfielder is seeking a new start after being left on the fringes of the Red Devils' first-team under Jose Mourinho, and it seems a good match between him and the Toffees, who are, after all, managed by his one-time Southampton boss, Ronald Koeman.

He made his debut during the weekend's massive win over Manchester City, coming off the bench in the 4-0 victory to get himself settled into the Everton first-team. His appearances are likely to rack up between now and the end of the campaign given the Merseysiders' investment in the French international, too.

There's every chance that his arrival at Goodison Park could be one of the best signings any club makes in this month's transfer window: the former Southampton man has a lot to prove and has a great platform to do that at Everton.

Here are FIVE reasons why Schneiderlin's arrival could prove to be an excellent bit of business by Koeman...

Potential partnership with Gueye

An Idrissa Gueye and Morgan Schneiderlin midfield could be one of the most effective in the entire Premier League, certainly in a defensive sense.

Both players excel at tackling and intercepting the ball. Gueye is already a dominant force in most matches he players but together they could be a near unstoppable force.

Everton fans will have to wait to see it though with Gueye currently in Africa representing Senegal in the Cup of Nations and not due to return for weeks.

By that time Schneiderlin should be well established in Koeman's starting XI and Gueye shouldn't take too long to settle back into life in the Premier League. It's an exciting prospect for the Goodison Park faithful.

His Southampton form

Schneiderlin may have struggled to make an impact at Old Trafford but let's not forget just how good he was at Southampton. It was his form at St. Mary's that got him his big money move to one of the world's biggest clubs, after all.

He was the catalyst for success and the Saints' rise from the English lower leagues to becoming an excellent Premier League outfit, along the way maturing into one of the best midfielders in the nation.

It was form that earned him an international call-up, too, making his debut for his country after six years on the South Coast and making the trip to the 2014 World Cup in Brazil.

If he can show that kind of progression at Everton then they're onto a winner and could potentially have a player on their hands that takes them from midtable to regularly competing for European positions.

His relationship with Koeman

Koeman will know how to get the best out of Schneiderlin after previously managing the player when they were both at Southampton for the 2014/15 season.

Together they achieved a 7th placed finish in the Premier League, a massive achievement for a club who were on the brink of not existing just a few years prior.

Koeman obviously trusts that Schneiderlin is the man to push his Everton side on and after the weekend's massive win over Manchester City they are well placed to go on an extended run of good form.

The Everton manager is finally putting his own mark on the squad and that's something that could usher in an era of success at Goodison Park.

Barry's replacement

Gareth Barry has managed to forge himself an exceptional career at Everton despite joining the club when he was 32. He's proven to be an exceptional Premier League midfielder even as his footballing career at the highest level draws to a close.

Schneiderlin could well prove to be an excellent long-term replacement for the former England international, as he possesses similar passing ability and boasts the same strong defensive attributes.

That could make his £20m+ move from Manchester United seem like a bargain considering Barry's importance to the Everton side since 2013.

The 35-year-old's contract runs until 2018 but we can't see it being extended past that. That 18 months should give Schneiderlin enough time to become the midfielder that Everton fans are hoping he will be.

The hunger

There's nothing like a frustrating spell at one club to motivate you to do the best work of your career at the next. We've seen it time and again with fringe players at clubs making their move, finding renewed confidence and going on to look as good as ever.

Schneiderlin never really got a fair shake at United under Mourinho, although he obviously had strong competition in the likes of Michael Carrick, Ander Herrera and Paul Pogba.

Everton gives him the platform to show United what they're missing and also the room required to cement himself as a first-team regular.

That hunger can only be a huge positive for Koeman and Toffees supporters.