Manchester United's thumping 4-2 derby victory over City is the latest vindication for the methods of manager Louis van Gaal. The Dutchman spent long periods of the season under fire for the disjointed performances of his misfiring team but is now basking in the glory of a dramatic upturn in fortunes. Convincing victories over Tottenham Hotspur, Liverpool and City have fans believing in Van Gaal's fabled philosophy. With a place in the top four all but assured and local bragging rights restored, here are five things the manager changed to transform United. Manchester United players celebrate Chris Smalling's goal during their 4-2 victory ove Manchester City . Here comes the Belgian . Who is this marauding midfield menace? Marouane Fellaini was public enemy No 1 last season, appearing to epitomise everything that was wrong with United under David Moyes. The scapegoat with the funny hair found it no laughing matter and this season has thrived under a manager who is utilising his strengths. Van Gaal is brave enough and bold enough to throw the big man into the action and give him licence to attack, regardless of aesthetics. If his unplayable form continues, expect sales of those daft wigs outside Old Trafford to rocket. Marouane Fellaini celebrates after the influential Belgian gave the Red Devils a 2-1 lead on Sunday . Fellaini leaps to meet Ashley Young's cross with a header to send Old Trafford into a state of euphoria . Last season, Fellaini struggled to make an impact at Old Trafford and was heavily criticised by fans . Ander-nother one comes good . Some United fans will tell you that they always knew Ander Herrera would do the business, he just needed the opportunity. Van Gaal however, will say that the box-to-box Basque boy is now thriving because he has taken on board his advice. The manager bemoaned a lack of composure from Herrera which saw him start just one Premier League match from October to mid-February. Since his return he has provided a cutting edge that United lacked until their spring rising. Van Gaal will no doubt claim similar credit for the resurgence of a reborn Ashley Young. Spanish midfielder Ander Herrera has been in impressive form for Louis van Gaal's side . The United boss congratulated Herrera after another good performance as United defeated rivals City . Figuring out what's the Mata . Juan Mata failed to live up to big expectations last season and looked like another who would prove to be a big-money flop. In fairness, it was a tall order to perform in a sinking side and some wondered if, given an apparent lack of pace, he would ever fit in. The man who arrived in a helicopter, however, finally has lift-off. Surrounded by willing runners he is now a key cog in a well-oiled machine. Loved by Chelsea fans, he is now close to being adored in Manchester. The Special Juan. Juan Mata slots the ball past goalkeeper Joe Hart to give his side a commanding 3-1 lead against City . Mata celebrates in front of the United supporters after firing his side into a two-goal lead . Reputation counts for nothing . Van Gaal has been dropping hints about his philosophy all season. The message came loud and clear when he was asked why he had preferred unproven teenager James Wilson to £280,000-a-week loan man Radamel Falcao. 'If a player cost £95m or £5,000 it’s not any different for me,’ Van Gaal told us. ‘You have to prove yourself.’ It is that mantra that has seen the likes of Falcao, £60m Angel di Maria and old pal Robin van Persie spend large parts of the season checking their backsides for splinters and has paved the way for the re-emergance of the likes of Mata, Herrera, Fellaini and Young. Radamel Falcao (right) has had to settle for a place on the bench after failing to impress Van Gaal . Record £60million signing Angel di Maria has also been made to sit on the bench by the United boss . Stumbling across a system . Despite snatching unlikely victories, United had laboured for long chunks of the campaign with the atmosphere at Old Trafford often flatter than a (Dutch) pancake. Not now. The three at the back is long gone and while it might not be the 4-4-2 the frustrated fans chanted for at QPR, the 4-1-4-1 (with a special mention for Michael Carrick) some would say the boss was forced into thanks to injury is not far off. By luck or design it is working. The Theatre of Dreams is rocking again and the nightmare of last season seems a million miles away. Michael Carrick has been key to United's recent form as his side move up the Premier League table .
Manchester United have won their last six Premier League games .
Louis van Gaal has made several changes since the start of the season .
Juan Mata, Marouane Fellaini and Ander Herrera have all impressed .
Angel di Maria and Radamel Falcao have struggled to make an impact .
READ: Van Gaal celebrates  derby win with CITY stars from ladies team .