The two strikers have been in fine form this season with United’s Swede banging in 10 goals in the last nine games.
Costa has also been sensational for Chelsea with 13 goals contributing to an 11 match winning streak for Antonio Conte’s side.
“I think Ibrahimovic is even more important to United”
However, Souness thinks Chelsea have greater depth than United who rely on Ibrahimovic for more moments of magic.
He told Sky Sports: “As important as Costa is to Chelsea, I think Ibrahimovic is even more important to United.
“I said at the start of the season, for United to have a season where they could be challenging, he has to stay fit and be their main man.
“I think he’s proving that.
“I’m also a fan of [Michael] Carrick. In that position I think there’s very few players who come anyway near him.
“He gets the ball and he’s always looking forward first.
“A lot of midfield players today are happy just to keep the ball and go square.”