Downing has done very well in his last few games for Boro, creating lots of chances through crosses. I don't see him having been injured over two months before England's first WC game being a problem. Beckham has repeatedly been the weakest link by a distance whenever he's played for England for the last few years, he always looks like getting himself sent off because he goes nuts whenever he gives the ball away and every team in the world has him sussed - stand in front on his right hand side and watch him go round in circles like a duck in water with one leg before passing the ball backwards 20 yards. Downing would still be the weakest link in midfield to an extent, but he'll improve the balance, not waste most of the free kicks, corners and not get himself sent off.
As for the forwards you primarily need someone who can win the ball in the air and just basically be an annoyance and distraction when they're partnered with Owen. I don't think Sheringham will be able to do this as well as Crouch, if Owen wasn't playing then it would all be different.
Having said this I hope Owen learns to anticipate Crouch's flick-ons a lot better, because Crouch almost always gets his head to an aerial ball and flicks it on - last few England games I've seen Owen stands still like a pillock expecting a pin point cushion header.