Wayne Rooney crossed for Ronaldo to volley home and spare United's blushes.
Ronaldo hit the post in the first half but United had debutant keeper Ben Foster to thank for twice keeping out Kenny Miller just before the break.
After scoring the goal, Ronaldo skipped clean through - only to be denied by former United team-mate Roy Carroll.
Another goal would have been harsh on Derby, who reacted to a 6-1 drubbing by Chelsea with a performance of dogged resilience against the league's top scorers.
United, meanwhile, were hoping to bounce back in style after going out of the FA Cup to Portsmouth - a match in which they spurned numerous chances.
But for 75 minutes, it looked like it would be a recurring nightmare at Pride Park.
They dominated the first half-hour and carved out their first good chance after only 59 seconds when Ryan Giggs headed over Ji-Sung Park's cross.
Ronaldo, playing alongside Rooney up front, hit the post from 12 yards before curling a shot with the outside of his right foot, which keeper Carroll did well to claw away with his fingertips.
Foster did not have a save to make in that period - although a couple of scuffed kicks were perhaps evidence of his debut-day nerves.
But the keeper had a chance to show his class when Derby twice threatened to take a sensational lead against the champions through Miller.
Foster first dived to his left to claw away Miller's volley from an Eddie Lewis cross then went the same way to stop the striker curling in a shot after Rob Earnshaw's through-ball.
Ronaldo continued to be United's main threat but the Portugal international made no attempt to hide his growing frustration as the ball refused to go into the net.
Carroll saved an audacious 40-yard free-kick, then a header and then another set-piece before Ronaldo lashed a shot into the side netting - prompting him to scream towards the sky.
But the breakthrough when it came was simple enough, Rooney muscling his way down the left before crossing for Ronaldo, who did well to steer his volley home with the ball at a difficult height.
Ronaldo's fervent reaction said it all, his goal putting United a point above Arsenal ahead of the Gunners' match against Middlesbrough later on Saturday.