Carlos Tevez made his expected debut for United, while highly-rated Portuguese youngster Nani was given a full debut after appearing as a substitute in Sunday's goalless draw against Reading, replacing Frenchman Mikael Silvestre - but there was still no sign of fellow newcomers Owen Hargreaves or Anderson in Sir Alex Ferguson's squad.
Nani and Tevez were both involved in the action when United took a 15th-minute lead through Paul Scholes after a crisp attack.
After halftime it was only the agility of David James that stopped United bursting 3-0 clear inside a few minutes.
The keeper knocked away Nani's shot with an outstretched leg after Ronaldo's pass set up the winger, then tipped over Tevez's blast after Scholes again made the running.
But it was Pompey who raised the roof with Benjani bagging a dramatic equaliser with a bullet header from Matt Taylor's cross in the 53rd minute.
It was the second goal in five days for the Zimbabwean, who has been criticised for his lack of marksmanship.
In a dramatic finish, Sully Muntari was sent off for what looked like a second bookable offence - a late challenge on Rio Ferdinand - and in the 83rd minute Ronaldo was red-carded for an suspected head-butt on Richard Hughes a minute later.