Pre-season is a very important time for Premier League teams. 

Some have used the time before the upcoming season to boost their squads in the hope of challenging for the title. 

The likes of Manchester United, Arsenal, Chelsea, Liverpool and Manchester City have all bought big this summer. 

Romelu Lukaku to United and Alexandre Lacazette to Arsenal stand out as two of the biggest transfers, along with Kyle Walkers move to City and Tiemoue Bakayoko's to Chelsea. 

As well as changing personnel, the off-season also gives teams the chance to change tactics during training and friendly games. 

All of England's top teams have jetted off across the world for pre-season tours, where they're set to take on each other and some of Europes best.

The two Manchester rivals will meet in the United States, while the Gunners take on Antonio Conte's Chelsea in Asia.  

The likes of Inter Milan, Bayern Munich, Real Madrid and Barcelona are also involved in these tours and some will take on these English sides. 

Of course, there's nothing on the line in these matches, so it gives managers the chance to experiment with their players, tactics and formations.  

United boss Jose Mourinho appears to be taking this chance, and he's hinted at the one thing he may copy from Conte's Chelsea this season. 

The Blues will be defending their title over the next year, after finishing seven points clear of second placed Tottenham in May. 

Much of their success last season was down to the manager and especially his shift in formation. 

Conte employed a 3-4-3 system last October, which led to his side winning a record 14 games in a row.  

And now, Mourinho has hinted that he may follow suit and use the same formation this season. 

After his teams 2-1 win against Real Salt Lake last night, the Portuguese manager suggested it's the system that makes all the difference. 

"It's the difference between playing with three in the back or four in the back," he told MUTV, as per Goal. 

“When you play with three, you project with the wingers and you need the central defenders to drive and to come with the ball."

"When you play with four in the back, if you project with the right-back and the left-back, you only have two defenders and they have to share [the workload] and keep their position more."

United's success in deploying a 3-4-3 system could be down to new signing Victor Lindelof. 

“His [Lindelof’s] quality of passing is always very good, he's technically very good but obviously with three in the back, he has more freedom to attack spaces in midfield."

The Swedish defender played 45 minutes of United's 2-1 win, but expect him to get much more game time ahead of August if Mourinho is serious about shifting formation.