The Portuguese winger blazed wide from 12 yards after Gabriel Milito had been penalised for handball, while Michael Carrick smashed a shot into the side netting with the Reds’ best second-half opening.
Samuel Eto’o wasted Barca’s best opportunity as a superb defensive display from Sir Alex Ferguson’s side neutralised the Catalans’ potent attack.
The prudent nature of the first leg could well have been blown out of the water as early as the second minute, however, had Ronaldo converted a golden opportunity from the penalty spot.
From a Wayne Rooney corner, Milito blocked Ronaldo’s diving header with his hands, and referee Massimo Busacca immediately awarded the spot kick.
After lengthy protests from the home side, the Portuguese winger placed the ball on the spot, adopted his trademark stance, sidled up and sidefooted his effort off target.
A huge let off for Barca, and a missed opportunity for United. Despite that glorious early opening, the game failed to descend into the anticipated slugfest where both sides would wantonly deploy their rich attacking arsenals.
With the stakes so high, the two teams opted instead to remain at arm’s length. Barca cagily probed with more attacking intent, while United reverted to the initial gameplan of containment and countering.