The Swedish legend completed a free transfer to Old Trafford from Ligue 1 champions Paris Saint-Germain in the summer transfer window.
Ibrahimovic scored a late winner in United’s 2-1 win against reigning Premier League champions Leicester City in the Community Shield at Wembley in August.
The 35-year-old centre-forward has already scored four times in seven top-flight games after finding the back of the net against Bournemouth, Southampton and Manchester City.
The former AC Milan and Inter Milan striker netted United’s winner in a 1-0 victory over Ukrainian side Zorya Luhansk in the Europa League last month.
Ex United midfielder Hargreaves believes Ibrahimovic has proven a shrewd piece of business given that the veteran striker arrived on a free transfer from PSG.
“The signing of the season by far has been Zlatan Ibrahimovic,” Hargreaves told BT Sport.
“To get him on a free transfer – and a player of that quality. I wouldn’t like to know where Manchester United would be without Zlatan Ibrahimovic.”
Ibrahimovic has won top-flight titles in Holland, Italy, Spain and France but has never lifted the Champions League.
The Red Devils have failed to qualify for Europe’s premier club competition in two of the past four seasons.
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