Porto's high-scoring attack could be missing its biggest weapon in forward Vincent Aboubakar for the Champions League round-of-16-match against Liverpool.
The Cameroon attacker didn't train with the rest of the squad on Tuesday because of a lingering muscle injury and remains doubtful for Wednesday's first leg at Estadio do Dragao.
He hasn't played in the last two matches for the Portuguese leaders, missing the derby against Sporting Lisbon in the Portuguese Cup and the league match at Chaves.
The 26-year-old Aboubakar has been in top form this season, scoring 20 goals in 28 matches.
He netted five of the team's 15 goals in the group stage of the Champions League. Aboubakar and forward Moussa Marega have combined to score more than half of Porto's 53 goals in the Portuguese league.
Yacine Brahimi is the other forward helping Porto thrive offensively this season. The team are unbeaten in their last 23 matches in all competitions, and only lost once at home -- 3-1 against Besiktas in the group stage of the Champions League.
Other Porto players nursing injuries include Marcano, Danilo and Andre Andre.
Porto, who have never beaten Liverpool in four previous attempts, are playing in the last-16 of the European competition for the second season in a row and are trying to reach the quarterfinals for the first time since 2014-15.
Five-time European champions Liverpool, looking to reach the quarterfinals for the first time since 2008-09, scored 23 goals in the group stage, but that was before selling playmaker Philippe Coutinho to Barcelona.