Teixeira had put Shakhtar ahead seven minutes after the break when 19-year-old Oliver Torres lost the ball in his area.
Shakhtar keeper Andriy Pyatov then saved a controversial first-half penalty from Yacine Brahimi, who scored an amazing hat-trick on his Champions League debut against BATE Borisov.
Shakhtar's Luiz Adriano seemed to have wrapped up the win after 85 minutes as he latched on to Bernard’s curled pass. But Porto struck back four minutes later through Jackson Martinez's spot kick awarded for handball by Yaroslav Rakitskyi.
The visitors' pressure paid off in added time with Jackson Martinez finding a perfect one-touch finish to Tello’s low cross from the left leaving the Lviv Arena silent.
"We deserved to have three points from this game but have only one instead due to our mistakes," Porto coach Julen Lopetegui told a news conference.
"But I liked the strength and mentality of our team. If we play like this, we can achieve a new level."
Shakhtar coach Mircea Lucescu was upset that victory had slipped out of their hands and cut the news conference short.
"I do not want to talk much. There was a great deal of euphoria and joy at the end of the game and then we conceded two goals... It is very hard,” he said.
Shakhtar have had to move their Champions League games to the western city of Lviv to escape strife-torn Donetsk. Representing the pro-Russian region of Ukraine, Shakhtar faced a big challenge to win the support of people in western Ukraine but there was a full house at the Lviv Arena with all the tickets sold out four days before the match.
(Editing by Ken Ferris)
