Erik ten Hag's side twice broke through the defences of home keeper and skipper Diogo Costa, through Marcus Rashford and Rasmus Hojlund, the Dane marking his first start of the season with a goal.
Back came the Dragons to level before the break, courtesy of headers by Pepe and Samu Omorodion and, shortly after the restart, Omorodion's second looked to be inflicting a disappointing defeat.
Bruno Fernandes was dismissed for the second game running, even if his red card against Tottenham was overturned, and with the visitors down to 10 men, all appeared lost. However, there was resolve shown to earn another draw in the competition, following the home clash with FC Twente, as substitute Harry Maguire again showed his aerial prowess from set-pieces with a header in the dying seconds to make it 3-3.
FIRST HALF – TOPSY-TURVY OPENING
You could not take your eyes off the first half, and it was the Reds who drew first blood after weathering the early storm. Porto pressed from the start, forcing Andre Onana into action inside a couple of minutes when Galeno fired in a shot, but were undone in only the seventh minute. Christian Eriksen fed Rashford and the left winger danced into the box with purpose. A low drive was too much for a weak right hand from Costa.
Boosted by some valiant blocks at the other end, Ten Hag's men continued to threaten going forwards and the Porto defence twice almost put through their own net in quick succession. Ze Pedro's touch to a Rashford cross looked like going in and, from the corner, a clearance cannoned off a defender and landed off target.
A few minutes later, it was 2-0 as Eriksen and Rashford set up Hojlund, again in the left-hand channel. The Denmark striker's shot was ineffectively handled by Costa and the ball clearly crept over the line before being hooked away. Another enterprising run by Rashford led to Costa's strong hand diverting away, but United were looking good.
However, the mood turned when Noussair Mazraoui headed towards his own goal, forcing Onana into evasive action. The ball dropped for Pepe, who was alert to the rebound, and the Brazil winger halved the deficit. Seven minutes later, Omorodion planted a stooping header high into the net by ensuring he was first to a teasing centre by Joao Mario and we were back to square one.
Despite the shift in momentum, Amad had an attempt that deflected out for a corner, Costa smuggled a Casemiro effort behind after Hojlund got a touch to a Bruno Fernandes cross and yet another bold dribble by Rashford unfortunately ended with him being crowded out when electing to shoot.
It was entertaining, yet frustrating, as we entered the interval with the score level.