Welbeck headed home from a United corner on 20 minutes.
"He won," conceded Ramos. "It was not a corner, it is true, but do not blame any player. I'll rely on the good attitude of the players. The margin of error is minimal in the Champions League.
"We'll go to Old Trafford to win.
"In front of us there is a great opponent that we must have the utmost respect."