The first-team squad - minus those involved in Euro 2016 and Argentinian Ramiro Funes Mori, who played in the Copa America - were scheduled to begin on Thursday.
However, Koeman brought them back today as he felt they had had a long enough holiday and he wanted to give him and them the best chance of starting the new season well.
"I'm very much looking forward to getting down to work," said the Dutchman, who will be seeing most of the players for the first time having only been appointed Roberto Martinez's successor last month.
"I'm extremely full of energy after a long summer and the players have had a lot of holidays as well.
"That's one of the reasons I decided to change the date for starting the pre-season because it was planned in for July 7 and we changed it to July 4 because I think the best pre-season is five or six weeks of planning.
"After a long holiday, the boys need to work and to get back fit to play football.
"With a new technical staff, we need to show them how we will work together.
"In pre-season, you get time to know all the players, to know the young players and, okay, you have the training sessions, but around the training sessions you have time enough to have a coffee with them and to have a talk to them.
"Of course, they need to know the manager, they need to know the rest of the technical staff and we need to know the players, the characters of the players and that's important
You get that time more in pre-season.
"We will look at organising in the training sessions, we will have some good friendly matches and it's all about preparing the team for the first Premier League game."
Source : PA
Source: PA