Joint leaders of group G, Chelsea and Porto, faced off in a match that could see both of them make it into the last 16 if the played a draw, or top spot for the winner and Europa league for the other team.
Chelsea, who are struggling domestically did everything possible to avoid meeting the top teams in Europe and secured a 2-0 win over Porto.
Mourinho watched on as his former team lost and were subsequently sent into the Europa league.
An early own goal, which was a deflection from Costa’s saved strike, put Chelsea in the lead.
Porto threatened, but the defense held firm and Chelsea settled matters when Willian doubled the lead seven minutes into the second half.