Richard Dunne believes the five Arsenal forwards looked ‘weak mentally’ at FC Porto with Kai Havertz and a ‘scared’ Gabriel Martinelli singled out for their lack of impact. The Gunners fell to a 1-0 defeat at the Estadio do Dragao this Wednesday night to trail after one leg. Porto visit Emirates Stadium for the reverse match of the Champions League last 16 tie on March 12. But Arsenal will rue Martinelli losing possession to help Porto take an edge. Galeno struck in stoppage time to claim the aggregate advantage with a superb finesse into the far corner of David Raya’s goal. Yet the Brazilian’…