Alonso led Leverkusen to a 4-0 Bundesliga win over Schalke in his first match in charge on Sunday (AEDT), but endured a more chastening experience against the Portuguese outfit.

It was a tale of three penalties at the BayArena, as Porto's Mehdi Taremi added to Galeno's opener with two second-half spot-kicks after visiting goalkeeper Diogo Costa denied Karem Demirbay from 12 yards.

Leverkusen has now failed to score any of its last three Champions League penalties, two of which have come against Porto this season (also Patrik Schick on October 4).

Despite the result, the Bundesliga outfit managed 64 per cent possession and 17 attempts in a front-foot display, leaving Alonso to describe the defeat as unjust. 

"We have to analyse the game, come back tomorrow and move on. Today there is pain, but that's football. Today is a lesson for us," he told UEFA.com.

"We could see today that the small mistakes can be fatal in the Champions League. It's hard for me to talk about the result because I think we didn't deserve it."

The former Spain midfielder became the seventh different Leverkusen coach to lose his first Champions League match in charge, but the first to do so by a three-goal margin.

Goalkeeper Lukas Hradecky also thought Leverkusen were hard done by, adding: "It was an unlucky game. It started in a bad way for us and ended in a bad way. 

"Shortly after the break we lost the second goal. Small details were decisive. At this point we have to work hard for success. It's a hard time for our club, but we will work on it."

Leverkusen remains bottom of Group B after its third defeat in four Champions League games this campaign, three points adrift of second-placed Porto.