Ronaldo was shown a straight red card by referee Steve Bennett four minutes from time after an apparent head butt on Portsmouth substitute Richard Hughes.
Last season's PFA Player of the Year now faces an automatic three-match ban, ruling him out of Sunday's Manchester derby.
Referee Bennett had an eventful night, having earlier dismissed Portsmouth home debutant Sulley Muntari for two bookable offences.
Ronaldo's red overshadowed Carlos Tevez's long-awaited United debut following his protracted move from West Ham.
The Argentine made an instant impression on his adoring new fans by creating Paul Scholes' 15th minute opener.
Tevez did brilliant to hold off the challenge of Noe Pamarot before playing the ball into the feet of Scholes who smashed it into the bottom right corner from 25 yards.
United, who also handed Portuguese winger Nani his full debut, threatened to run riot in a one-sided first half.
Tevez should have got himself on the scoresheet in the 35th minute when David James spilled a Ronaldo shot into his path but the 23-year-old shot wide.
Pompey boss Harry Rednapp brought on Matt Taylor and former Liverpool defender Djimi Traore in a double half-time substitution.
But it was the visitors who again came out of the blocks quickest with James pulling off a superb save with his legs to deny Nani his first goal in English football.
Tevez wasted another gilt-edged opportunity in the 52nd minute when he latched on to Scholes' clever chipped pass only to lift his shot over the bar from six yards.
The South American was made to pay for that blunder when Benjani drew Pompey level with his second goal in as many games.
Substitute Taylor whipped in a teasing cross from the left and Benjani expertly headed into the top corner.
James was called into action again in the 68th minute to chip a Ryan Giggs header over the crossbar.
David Nugent should have been celebrating his first Premier League goal 14 minutes from time after being played in by Benjani, but fell embarrassingly on his backside with the goal gaping.
Muntari, booked ten times and sent off on three occasions for Udinese last season, was sent for an early bath seven minutes from time after picking up a second yellow for a late lunge on Michael Carrick.
Ronaldo quickly followed the Ghanaian four minutes from time as the match descended into farce after a 20-man man brawl over a corner.