No Premier League manager knows Marouane Fellaini better than David Moyes.
The Manchester United boss wouldn't have agreed to the expenditure of a cool £27.5m on the towering Belgian midfielder if he wasn't absolutely sure his former star man from Everton could cut the mustard at Old Trafford.
Moyes surely anticipated that Fellaini would require time to settle with the reigning Premier League champions. Shining bright for Everton is one thing; but consistently producing seven or eight out of ten performances for the Red Devils is quite another.
"He's not had the best of starts," Moyes admitted this week. "But this is a difficult club to join and settle in right away. It's a new manager as well, so from that point of view, he can take time to settle in.
"But he is a good player, and has been for a few years at this level, and I have no doubt he will go on to prove it with this club."
The match-going Manchester United supporters are a patient bunch - but they will be desperately hoping that Fellaini will prove his worth and justify that huge price tag, starting tonight against his old club Everton.
With Michael Carrick out injured and Phil Jones suspended, Moyes is expected to hand Fellaini a start in his favoured central midfield position against his former employers.
The 26-year-old needs a big performance - for himself, more than anyone - and a talismanic display would instantly endear him to the Manchester United faithful.
Moyes' side are currently eighth in the Premier League table - nine points off league-leaders Arsenal - and one of the main reasons for their mixed results has been down to a lack of control in midfield.
Fellaini is not responsible for this - he has only featured in a handful of Premier League matches this season. But he could well be the solution.
The Belgian has the quality to dominate the middle of the park. All he needs is a few good performances to boost his confidence and, ideally, a goal.
And what better time to open his account for his new club than tonight.
"The United fans have not seen the best of him," Phil Neville, who played alongside Fellaini at Goodison Park and now coaches him at Old Trafford, revealed this week. "When he came he wasn't fully match fit but we are getting him fitter to play for United and be that box-to-box player.
"But we have every faith in him and we are really relaxed about it.
"What is important for Marouane now is to play well in a match we win and he scores a goal.
"He needs to experience that feeling when you are part of a team that wins and he needs to start scoring from midfield. There is nobody better at that than Felli."
Those who know him best at Old Trafford clearly have the utmost confidence in his abilities.
Now Fellaini must show that he has the utmost confidence in his own talent.