Another capacity crowd turned up at Old Trafford expecting to witness Welshman Ryan Giggs make his 1,000th senior appearance for club and country but it was newcomer Kagawa, still a one-year-old when Giggs made his United debut in 1991, who stole the show as United romped 15 points clear.
He put United ahead on the stroke of half-time and scored two more goals after the break before Wayne Rooney's late screamer rounded off a victory that kept Sir Alex Ferguson's side surging relentlessly towards a 20th English league title.
Despite the proximity of United's Champions League last 16, second leg clash with Real Madrid on Tuesday, they fielded a strong line-up although Giggs was given a rare afternoon off.
Champions Manchester City can close the gap to 12 points with victory at Aston Villa on Monday but the most exciting issues left this season, barring a United collapse, appear to be the battle for the top four and the relegation scrap.
Chelsea leapfrogged Tottenham Hotspur into third place with Demba Ba's first-half goal earning under-fire manager Rafa Benitez's side a 1-0 win at home to West Bromwich Albion.
Interim boss Benitez, who criticised the club's fans in the week when he confirmed he would leave the club in May, was spared the anticipated barracking from the stands as Chelsea moved a point above Spurs who faced Arsenal on Sunday.
"There were a lot of positives. The reception from the crowd was quite good. I wanted everybody to stick together," Benitez, taunted with one banner declaring him "The Interim One", said.
"You can see today the fans were behind the team."
Luis Suarez scored a hat-trick for Benitez's former club Liverpool in a 4-0 win at Wigan Athletic to move above Manchester United's Robin van Persie in the Premier League scoring charts with 21 and maintain his side's top-four hopes.
Everton also revived their chances of qualifying for the Champions League with a 3-1 defeat of visiting Reading to move to within six points of Tottenham - Marouane Fellaini, Steven Pienaar and Kevin Mirallas all on target.
Bottom club Queens Park Rangers capitalised on Reading's defeat, winning 2-1 at Southampton to leave Harry Redknapp's side four points adrift of fourth-from-bottom Wigan.
League Cup winners Swansea City avoided a Wembley hangover with a 1-0 home win over Newcastle United, Sunderland recovered from two goals down to draw 2-2 with visitors Fulham and West Ham United eased relegation fears with a 1-0 win at Stoke City.
Kagawa, whose first season at Old Trafford since signing from Borussia Dortmund started promisingly with a goal on his home debut but who has since been disrupted by injury, delighted Ferguson and the large United fan club back in Japan.
"It's brilliant for him. The lad is a good finisher and his second goal was so compose
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