Porto dominated throughout in Switzerland and deservedly equalised courtesy of Danilo, whose 79th-minute penalty cancelled out Derlis Gonzalez's first-half opener.
Basel were outplayed for the entire match at St Jakob-Park, but their only meaningful attack resulted in a goal, Gonzalez finding the back of the net in the 11th minute after poking the ball beyond Porto goalkeeper Fabiano with the outside of his boot.
Porto had two goals ruled out in search of an equaliser but the visitors were not to be denied after an avalanche of attacks as Danilo converted a spot-kick 11 minutes from time following a handball inside the penalty area.
While heaping praise on Porto, Sousa insisted the Swiss champions head to Estadio do Dragao on March 10 with hope.
"Conceding that goal has complicated things for us ahead of the second leg, but I am satisfied with the result. Porto were superior to us," Sousa is quoted as saying by UEFA.
"They may not have had any clear-cut chances, but they deserved to equalise and perhaps even more than that. Porto have outstanding individual talent.
"However, we can hold our heads high going to Portugal, and we can certainly improve.
"There were spells in the match when we may have been a bit too afraid. I know we can do better. Now we are travelling with a vision and with hope."
