The Red Devils have lost the previous four Manchester derbies, and Van Gaal hopes a rousing speech from the influential striker will inspire a positive result this weekend, according to The Sun.
It would be the second time this season that Rooney has conducted a speech in front of his teammates. His first came last month, ahead of United's comprehensive 3-0 win over Tottenham, which appears to have been the turning point in Van Gaal's tenure at Old Trafford.
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United the narrow favourites against City
Manchester United have since recorded similarly convincing wins over Liverpool and Aston Villa - and there's every chance the Red Devils will beat the reigning Premier League champions on Sunday. The bookmakers currently have third-placed United as the narrow 6/4 favourites, with City priced at 9/5.
Manuel Pellegrini's side have dropped to fourth in the Premier League table following three defeats in their previous five fixtures and will be looking to get their campaign back on track with a morale-boosting victory over their rivals.
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United the in-form side
But United are unbeaten in the league since mid-February and have not lost a Premier League match at Old Trafford since January 11.
Van Gaal's side suffered a 1-0 defeat to City back in November, with Sergio Aguero scoring the only goal of the match following a red card to defender Chris Smalling earlier in the match.
Despite the result, United performed reasonably well on the afternoon, and it was a markedly improved performance from the previous season's 4-1 defeat at the Etihad Stadium.
Rooney's excellent record against City
Rooney scored the consolation goal that afternoon - a magnificent free-kick placed to perfection past Joe Hart - which was the 11th goal the England skipper has scored against Manchester City in his career.
The 29-year-old remains the driving force behind Manchester United, the team's heartbeat, but others have stepped up to the plate in recent weeks - in particularly the Spanish contingent.
Ander Herrera and Juan Mata have been superb in recent weeks and both midfielders could have a big role to play this weekend, along with goalkeeper David de Gea, who will be looking to rediscover his best form after two errors in as many matches.
Will Manchester United beat Manchester City this weekend? Post your score predictions in the comments below!