A blunder by Poland goalkeeper Lukasz Fabianski, deputising for the injured Manuel Almunia, had gifted Porto an early lead, only for veteran Campbell to head in what was his first Arsenal goal since his strike in the 2006 Champions League final, after he rejoined the Gunners on a short-term deal in January.
However, the 35-year-old former Portsmouth skipper was then involved in a bizarre incident at the start of the second half when Fabianski picked up what was deemed to be a back-pass from the centre-half and referee Martin Hansson allowed a quick-free kick which Falcao swept into an empty net.
With the tie delicately poised, Campbell - who hopes to recover from a groin problem for Tuesday night's European encounter - believes the away goal could yet be decisive.
"The game is only halfway done, and that should stand us in good stead for the second leg," he said.
"Scoring away from home was important. Had we lost 1-0, it would have been harder. That one away goal could make a massive difference.
"It gives us a bit of an advantage to build on for the second leg.
"But, we are not happy because we lost the game.
"What we have to do now is take the positives from the game and go again as a team.
"There is still a big job to be done, don't get me wrong, but it would be much harder without that goal."
Gunners boss Arsene Wenger was less than impressed by the way Swedish official Hansson allowed the free-kick to be taken so quickly, with no chance to set up a proper defence.
Campbell also felt Hansson had been a touch hasty in letting play restart.
"I did touch the ball, but it was not intentional, so I guess it is down to the referee whether he gives it," said the former England defender.
"For the free-kick itself, the referee did not step away from the ball.
"You can go on about it, and yes it was quick thinking from the Portuguese, but the ref has never given me a chance to get in front of the ball - basically he gave it to them and let them play.
"There was no whistle and I do not think he even had his arm up when the kick was taken."
Arsenal have played Porto twice in recent campaigns, romping to a 4-0 victory at the start of last season while also winning 2-0 at Emirates Stadium in September 2006.
Captain Cesc Fabregas sees no reason why the young Gunners cannot produce another impressive display in north London.
"We knew we would not go to Portugal and win 3-0, we expected a tight and difficult game, but when we got back to 1-1, we were controlling the game," he said.
"Of course we do not want to concede the type of goals we let in, and if you take those mistakes away, it means that they could not score from open play."
Fabregas added: "The positive for us was the away goal - I am still confident we can go through.
"In the Champions League, you can get through playing well or sometimes not playing well.
"It is just one more game and we will have to win by at least a goal.
"I am sure we have the quality to do that against them - we have done it before at the Emirates, and we will have to be at our best."
Arsenal vs Porto To Qualify: Arsenal 4/6