The Red Devils have been constantly linked with a move to bring Sancho to Old Trafford from Borussia Dortmund this summer but a deal has so far not materialised.
With the summer transfer window set to close on Monday at 23:00 BST in the Premier League, time is running out for the Red Devils to strengthen their squad.
Despite Manchester United having been relentlessly linked with Sancho this summer, former Red Devils star Rooney feels that the club would have been better off focusing their attentions on trying to sign Kane, 27, from Spurs.
Writing in his column for The Sunday Times, Rooney said: “Jadon Sancho is a top player, but I find it odd that he has been Manchester United’s transfer priority.
“Why consider paying close to £100m for someone similar to talent you’ve got already? United have Marcus Rashford and Anthony Martial and where would Sancho’s arrival leave Mason Greenwood, who plays in the same position and is of similar age?
“I’d rather put that £100m towards trying to get Harry Kane.
“Yes, getting Kane out of Tottenham would be difficult, maybe impossible, but United are a club who should always make an effort to sign the very best players and Kane is exactly what Ole Gunnar Solskjaer needs, as is Erling Haaland, who United missed out on last year — a genuine number nine.
“A Kane or Haaland or someone like Robert Lewandowski would give Rashford and Martial someone to play off and freedom to roam into different areas, pick the ball up and try to beat men — without the burden of always having to score.
“It would be better for him as well if United had a recognised number nine, because he could learn from them, just as I did from Ruud van Nistelrooy. Of course there are different ways to play the position and Liverpool make brilliant use of a false nine in Roberto Firmino.
“I wonder if a solution for Solskjaer could be a diamond midfield with Bruno Fernandes at the tip and Martial and Rashford as wide forwards, using their speed, similar to Mohamed Salah and Sadio Mane.”
Kane scored 24 goals in 34 games for Tottenham Hotspur last season as they finished in sixth place in the Premier League table.