A UEFA spokesman said: "It is mentioned in the referees report so we will open a disciplinary case against Porto.
"Usually it is the home team that has to control the fans, but teams can be fined for the behaviour of their fans in away games. The punishment will probably be a fine."
The fan who spat at Mourinho was ejected from the ground and hosts Chelsea are not expected to face any repercussions.
Mourinho, who guided Porto to European Cup triumph last season, said: "I can understand it. Some people love me, and others don't love me that much. That's just life."