The visitors took the lead at Estadio Santiago Bernabeu when Welbeck beat Ramos in the air off a corner kick and directed his header past Real Madrid goalkeeper Diego Lopez.
While the 26-year-old Ramos admitted it was his costly mistake which led to the goal, he said the corner should have never been awarded to begin with.
"He got the better of me but it was not a corner in the first place, that is certain," Ramos told reporters. "The margin for error in the Champions League is tiny."
Nonetheless, the Spain international was buoyed by the performance of Los Blancos on Wednesday and insists that although the club never got the result it wanted, there is cause for optimism heading into the second leg.
"We would have liked to be going to Old Trafford with a different result but we have to be optimistic and appreciate the great game Real Madrid played today even in spite of the fact that we missed a lot of chances," Ramos told Uefa.com.
"On some days, clear opportunities just don't translate into goals but every player put in a huge effort. We will have to travel to Manchester and show the same mentality as we did today while believing that we can win. We have to take into account that we are facing a great rival; we need to show the utmost of respect to them. The main thing is that we convince ourselves we can win."
The second leg is scheduled to take place at Old Trafford on March 5.