The versatile winger played two and a half seasons under Mourinho and became a club favourite with his dynamism and prowess. United finished sixth in the Premier League and weren't much of a threat domestically.
And the United manager has pointed out that his remarkable record of winning the domestic title in the second season at his previous clubs offers no guarantees that the trend will continue this term.
"I have conditions to deliver a better team than I did in the first season".
Manchester United's opening six matches are against West Ham, Swansea City, Leicester City, Stoke City, Everton and Southampton and a strong start is a must for Mourinho and his men.
"I heard whispers at the back end of last season that he wanted to go and even if the club can not come out and say too much about that, maybe there was an element of that in play here when this deal went through".
"He had a good fight against two good central defender and I am happy with this period and how he fits into - I am never exhausted to repeat - an wonderful group".
The Sweden global was United's top scorer last season, hitting 28 goals as they won the Europa League, the League Cup and the Community Shield.
"It is hard job to play against a team that dominates the possession - and he tried", Mourinho said of his striker.
"We had three players as you know, one defender, one midfielder and one attacking player but for this beginning of the season we lost important players too, we lost [Marcos] Rojo, [Ashley] Young, [Luke] Shaw, Zlatan, we lost players for injuries so I think between the players we lost and we got we are in a similar situation", he said. "They have to play on the Thursdays, but the top seven are going to be in the four competitions, so no advantage for any one of us". I worked with Mourinho and I became his friend as well.
Matic adds aggression and discipline to United's midfield and he will fit right into Mourinho's preferred three-man midfield and should be a regular figure in the starting XI alongside Ander Herrera and Pogba.
"It's different to any one of the other leagues". If the remaining 25 per cent doesn't arrive, I will not moan.
"They have improved the squad with quality and big transfers, so we will have to be on top of our game".