Terry sprained an ankle in the opening minutes of the game, and before Jose Mourinho could bring on a substitute, Raul Meireles fired the Portuguese side ahead.
Andriy Shevchenko grabbed an equaliser though to leave Chelsea as favourites to progress to the quarter-finals of the competition.
Mourinho, who had coached Porto to Champions League glory in 2004, was given a warm reception by the home fans on the first anniversary of Chelsea's 2-1 defeat to Barcelona as the Blues crashed out of last season's competition to the eventual winners.
Chelsea started brightly and in the fourth minute the unmarked Michael Ballack skimmed a header wide.
However, after 12 minutes and with Terry off the pitch, Meireles tried his luck from the edge of the area after good work by Helder Postiga, and a deflection off Frank Lampard deceived Petr Cech as the ball nestled in the back of the net.
Mourinho made a bold move by bringing on Arjen Robben for Terry and almost immediately the Dutch winger set up Shevchenko, who drilled a shot past Arruda Helton and into the net.
It was the perfect response by the visitors, just when it looked as though everything was conspiring against them.
Porto should have made it 2-1 midway through the first half but Cech made a fine save to deny Lisandro Lopez who looked certain to score after being set up by Postiga.
At the other end Didier Drogba headed Lampard's lofted free-kick wide when the Ivory Coast international should have at least hit the target.
The impressive Ricardo Quaresma then forced Cech to sprawl along his line to make an untidy stop from his free-kick before the same player beat the Czech Republic shot-stopper with a brilliant effort that came back off the crossbar.
Postiga guided a snap-shot straight into Cech's midriff to end an open and entertaining first half.
After such a promising first 45 minutes, the second period was virtually devoid of interest after Mourinho opted to take Robben off and bring on John Obi Mikel.
Both sides lost their rhythm in a disappointingly sterile second period. Porto appeared to become disheartened as they struggled to break through the Chelsea rearguard, and it was the Premiership side that went closest to snatching a winner in the closing stages.
With 12 minutes left Ballack and Shevchenko set up Drogba, but his first touch was poor and from a tight angle the striker lashed a shot against the post.
Shevchenko, who enjoyed a good game, then did well to find space for himself, but when he fired a cross in to the danger area, Drogba sent a weak header wide.
After good build-up play, Lampard then forced Helton to make a smart save down by his post, but with virtually the last kick of the game Porto almost snatched a dramatic winner.
Lopez beat Chelsea's offside trap, rounded Cech, but then contrived to fire the ball over the bar.