While United sit third in the Premier League table after just one defeat from their past 15 matches, the Manchester outfit have failed to convince.
United were completely outplayed by relegation-threatened Burnley during last week's 2-1 victory at Old Trafford.
That performance was met with whistles and chants of "Attack, attack, attack" by United supporters, something that concerned Van Gaal.
Van Gaal was also heavily criticised by iconic United midfielder Paul Scholes, who said the club were obsessed with possession and miserable to watch.
And Docherty - winner of the 1977 FA Cup and 1974-75 Second Division title during his five-year stint in charge of United - has joined the growing list of critics, claiming Van Gaal is running the risk of alienating the fans, despite his "fantastic" record abroad.
"Once the crowd start to get on you if you don't give them what they want – and they're the ones who pay your salary – they'll start showing disillusionment with the way the team is playing," Docherty told The Mirror.
"The fans at United have been brought up with great football – two wingers going down the touchline, raining crosses in to the strikers. That's not happening.
"You've got to give the supporters what they want, and you can't really see a lot of progress at the moment, with the money he's already spent, let alone the money he's set to spend this summer.
"With that, you have to guarantee success. They've still got a good chance of finishing in the top four and back into the Champions League, but the way they're playing there's no guarantee.
"If United get beaten by Preston [in FA Cup fifth round on Monday], their season's almost over. Then all they'd have to go for is the top four – and they've got to get into that.
"A bad season for United, with the money they've spent and the players they've got, would be finishing in the top-four. They've got to get into the top four every year – minimum.
"If David de Gea, who has shown he's world-class, hadn't played the way he has this season, United would probably be in the relegation zone now. He's been absolutely brilliant."
