Marega, filling in for the injured Vincent Aboubakar up top, netted 22 goals in Primeira Liga last term, but doubled his tally for this season when he took advantage of abysmal marking to head home a corner from former Galatasaray man Alex Telles.

Things would have been different if not for Casillas, however, with Porto's goalkeeper pulling off two superb saves towards the end of the first half.

Casillas' clean sheet should have been ruined late on, but both Rodrigues and Sinan Gumus failed to convert good chances as Porto held firm to move top of the group.

Galatasaray started well and could have had the lead when Henry Onyekuru picked out Gumus, but Maxi Pereira was on hand to make a vital block.

Porto rallied, though, Jesus Corona teeing up Yacine Brahimi, whose acrobatic attempt was brilliantly kept out by Fernando Muslera.

Not to be outdone, Casillas was in equally fine form at the other end, Porto's goalkeeper denying both Yuto Nagatomo and Gumus in quick succession.

Marega made those saves count four minutes into the second half, easily getting the better of Galatasaray's static defence to nod in from Telles' delivery.

Rodrigues blazed over from close range and Gumus headed wide as Galatasaray searched for an equaliser, but it was Marega who should have had the final say with seven minutes remaining.

Muslera got down to deny Marega a second, though the miss ultimately mattered little as Porto got its first Champions League Group D win on the board.