It may turn out to be a serendipitous twist of fate, with Lukaku bringing proven Premier League experience and a haul of 85 goals over the past five seasons.
And while he says Lukaku can be a new attacking threat for United, he must work on his first touch and become more consistent if the Old Trafford club is to compete.
However the club's all-time appearance holder and most decorated player has concerns about the 24-year-old's touch and ability to deal with the pressure.
"Step by step, step by step", Mourinho said of the centre half.
The flexibility in formations we've seen in preseason will be key to navigating the Premier League and Champions League campaigns but at this stage United do not look ready to win either.
"I think we have a chance, but in football until things are official, you never know".
Just six players to taste Premier League success were left in the United ranks after Wayne Rooney's departure earlier on this month, which is unbelievable to think, so Matić's experience should be a welcome asset going forward.
IN: Victor Lindelof (£30.75m, Benfica), Romelu Lukaku (£75m, Everton), Nemanja Matic (£40m, Chelsea). Whilst the fan in me is cautiously optimistic that we can win the quadruple (as I am every year), the realist says we need two more signings to be considered genuine contenders.
Lukaku has already hit the ground running at his new club. Romelu Lukaku will score goals, there's no doubt about that.
Even with Ibrahimovic's contribution, United were still the lowest scorers in the Premier League's top seven last season.