United manager David Moyes predicted Sunday that Rooney would be out for "probably three weeks."
"It's bad," Moyes said. "It's right down the middle of his forehead and it will take a while to heal. He turned round and Phil Jones just lifted his boot as his head came in."
Rooney missed United's 1-0 loss to Liverpool in the Premier League on Sunday. England hasn't yet confirmed whether the striker has been ruled out of Friday's match at home to Moldova and the trip to Ukraine four days later.
England's ticket to the World Cup finals in Brazil is far from secure, with the team second in its qualifying group — two points behind Montenegro with a game in hand and with four matches left. Ukraine is a point further back in third.
England coach Roy Hodgson would also have been concerned to hear Liverpool manager Brendan Rodgers say that striker Daniel Sturridge, who scored the winner against United, is also struggling for fitness and was only playing at "70 per cent" at Anfield on Sunday.
The other strikers in England's squad are Rickie Lambert, Jermain Defoe and Danny Welbeck.
To add to Hodgson's problems, right backs Jones and Glen Johnson hobbled off with ankle injuries during the Liverpool-United match and are doubts.