Manchester United were one of five clubs that voted against the change at a meeting of clubs on Thursday.
The 2018 summer window will now close on Thursday August 9 at 5:00 pm, two days before the new Premier League season kicks off.
United manager is pleased with the move although he understands why his employers voted against the new deadline and he has expressed his fears about some clubs regretting the switch.
Speaking to MUTV Mourinho said: “As the head coach of Manchester United, obviously I am in favour of that because if I focus just on the training process of course.
“I want to arrive, day one, I want to have my squad, my players, I want to work and travel with them in pre-season, I don’t want to be waiting for late decisions.
“But there is another perspective, and I have to share the problems the board has, and that is that obviously when the decision is not uniform around Europe and all the championships then we are opening a disadvantage for the English clubs.”
Mourinho strongly believes that United will be strong enough to fight off interests in their players in the three weeks between the closing of the Premier League window and the pan-European deadline.
But he is equally sure that other Premier League clubs may not be so fortunate.
He said: “That was the reason why Manchester United voted against because there is another element – the market will be closed for us and is open for others and they can come to the English clubs, buy players, and they can leave English clubs without the possibility of a big action, so it’s another element of weakness.
“If you ask me about Manchester United, I hope we are strong and powerful enough. We also close our doors and say the offer doesn’t matter, we close the market for buying, for selling.
“I expect my club to be acting that way but there are other English clubs that will be in risk. I hope it goes to the perfect situation, which is to close for every championship and not just the Premier League.”