Struggling Manchester United star Marouane Fellaini was hailed as a game-changer and described as one of Europe's best headers of a ball after rescuing Belgium's World Cup opener against Algeria on Tuesday.

With tournament dark horses Belgium trailing to Algeria's first World Cup goal in 28 years, the former Everton midfielder came off the substitutes bench to power in a header to equalise in the 70th minute.

Fellow substitute Dries Mertens drove in the winner 10 minutes later to give Belgium a winning start in Group H.

"Fellaini is one of the best headers of a ball in Europe, he brings a lot to the game in terms of his physical impact," said Algeria's Bosnian coach Vahid Halilhodzic.

"He was so dominant in the air that my players didn't have an answer to it. He's an extraordinary player who changed the game."

Fellaini spent five years at Everton before signing for Manchester United in September last year in a £27.5 million ($46.6 million) deal.

But he made just 16 league appearances and failed to get on the scoresheet.

Belgium coach Marc Wilmots admitted that the second half introductions of Fellaini, Mertens and Divock Origi had been decisive in sparking a sluggish Belgium side who are many commentators' outsiders for the title.

"Mertens has speed on the wings, Origi has strength up front and Fellaini has physical force. This worked well," said Wilmots.

"Dries was dynamite on his wing and he has a nose for goal while Marouane was too physical for the Algerians."