Manchester United manager Jose Mourinho has suggested he couldn't afford to "gamble" on certain players this summer for what will be a pivotal second season at Old Trafford.

Mourinho got three of his four transfer targets during the summer window with Victor Lindelof, Romelu Lukaku and Nemanja Matic all arriving to bolster a squad that celebrated League Cup and Europa League successes last season.

While the Portuguese has traditionally enjoyed his biggest triumphs in his second seasons, the United boss believes he will need a third summer transfer window before he finally has a squad he believes will be ready to challenge Europe's elite. In an an interview with the Sunday Times, he explains three summers are the "ideal to put the team back on track, but also to make the whole structure solid and working like a perfect machine."

In putting together that "perfect machine," Mourinho says he has had to make some difficult choices over potential recruits who might not fit in with what he describes as a "special group of players."

"In the second season, we know that the team by the human point of view is a very special group of players. And that makes it more difficult because you cannot bring a very good player that is not a very good man. You cannot gamble."

He added: "In my career, the biggest achievements were when the group was a special group."

In Paul Pogba and Zlatan Ibrahimovic, Mourinho says he found two players who could provide the personality and quality he believes the club "lost in the past years," while in Eric Bailly and Henrikh Mkhitaryan he found two players with quality "I thought the team didn't have."

In Lukaku and Matic, he now has two he refers to as "his players," explaining how his decision to sell the former to Everton in 2014 did not affect their relationship.

"He [Lukaku] left the club because at that time he was thinking he deserves more opportunity from me, but he is such a good guy that during these years we were in close contact with a very good relationship because probably he understood that I couldn't accelerate his timings at that moment. So I knew he was an intelligent guy with a very positive attitude."

In the interview, Mourinho is glowing in his praise of Matic, commenting: "I cannot say that I had better guys than him in my career." The Serbian was a key figure during his title triumph with Chelsea during the 2014-15 season but was also involved in one of the more notable moments during the manager's doomed 2015-16 season in Stamford Bridge, which saw him sacked four months into the campaign.

In October 2015 as things began to unravel, Chelsea were being held 1-1 by Southampton at home with Mourinho bringing Matic on for the second half. Just 14 minutes later however his match was over, with the manager choosing to haul him off in place of Loic Remy. Chelsea went onto lose 3-1.

While Matic was disappointed with the decision, Mourinho was struck by how understanding he was of his manager's call, a moment which has stuck with him since.

"The press wants a story, the press wants blood. My blood or his blood or both," Mourinho said, recalling the incident. "He was really sad because it is not something nice and it something that I did only twice in my career. But the next day he comes to me and he says 'I'm not happy, but it's my fault. I'm not happy with what you did to me, but it's my fault, because the way I was playing I can understand the change. So let's keep going'."

Nemanja Matic
Matic has been a commanding presence in the United midfield since reuniting with Mourinho. Getty Images