Two errors from Arsenal goalkeeper Lukasz Fabianski allowed Porto to establish a 2-1 lead in the first leg of the UEFA Champions League last 16 tie on Wednesday.
The 24-year-old Pole started in place of the injured Manuel Almunia and gifted the hosts an early lead with a soft own goal. He then presented them the opportunity to score their second five minutes into the second period by inexplicably picking up a back-pass.
The industrious Falcao was the beneficiary on the latter occasion, rolling the ball past a stranded Fabianski after Ruben Micael took a quick free-kick to secure a slim advantage for his side prior to the second leg on 9 March. "Schoolboy goals. What can you do? Nothing. After the second goal, we went down," said Arsenal captain Cesc Fabregas. "We are still a little bit soft in that aspect. When we concede a goal we are not strong enough to lift ourselves up. Until the second goal we were having a very good game."
Fabianski's nightmare performance distracted attention from an impressive return to European action by Sol Campbell. The 35-year-old veteran marked his first appearance in continental competition since his return to the club in January by scoring the equaliser - three years and nine months since his goalscoring turn in Arsenal's defeat by Barcelona in the 2006 Champions League final.
Porto's winner sparked a furious reaction from Arsenal coach Arsene Wenger, who was also without William Gallas, Alexandre Song, Andrey Arshavin, Eduardo and long-term absentee Robin van Persie due to injury. Fabianski kept the score down with a save from Hulk, but Arsenal continued to look unsettled at the back and will pray that Fabianski recovers morale in time for the English Premier League visit of Sunderland on Saturday.
There was no gentle return for Campbell, who was exposed as early as the second minute, but having been sold short by a loose pass from Bacary Sagna he responded brilliantly to thwart Falcao with an outstretched right leg. Hulk flashed a shot wide of Fabianski's right-hand post barely a minute later, while Campbell was perhaps fortunate to avoid punishment for a strong shoulder barge on the advancing Hulk just inside the Arsenal box.
Disaster first struck for the visitors in the 11th minute, when Varela broke into space down the right before flinging a low cross towards goal that Fabianski, who had edged forward, weakly fumbled over his own line. Arsenal, though, were quick to react and drew level eight minutes later, Campbell heading home from close range after Rosicky had turned a deep corner back across goal.
Porto goalkeeper Helton was twice called upon before half-time, pushing away a drive from Rosicky and then athletically touching a Nicklas Bendtner header over the bar, while Fabianski repelled Micael's long-range effort at the other end.
"We played a good game against a difficult team," said Porto coach Jesualdo Ferreira. "We showed initiative and left Arsenal with no options. Our forwards adopted strategic positions. Arsenal dominated some parts of the game but it was a balanced game and we also had lots of chances."
Arsenal manager Arsene Wenger said: "I don't want to comment individually on Lukasz's performance. I do not want to judge him in front of everybody. We lose as a team and we win as a team. Porto played well but we have a chance to turn it around in the second game and I am convinced we will do it. Of course we hope that justice will be done."
Wenger was, however, enthused by the performance of former England international Campbell, who turned in a committed defensive display in his first Champions League game since Arsenal's 2006 final defeat by Barcelona. "He had a decent game," said the Frenchman. "He's disappointed by the second goal, but he scored an important goal that could prove to be vital in qualification."