The Latics have impressed going forward at times this season, but a 3-2 home defeat by Sunderland on Saturday highlighted their vulnerability at the back.
With Antolin Alcaraz missing almost the entire campaign so far with a calf injury and Ivan Ramis ruled out for the season's run-in following major knee surgery, the back three unit which proved so successful at the start of the season has been ripped apart.
"What is clear is that we weren't strong enough defensively," said Martinez whose team are one place off the bottom.
"If Ramis had been on the pitch it would have been a very different situation. I need to make sure we rectify that and bring someone in who will give us the same strength Ivan was giving us.
"Antolin is still injured and we need to see how quickly he is going to be available because it is very unfair that the players who are here are having to cope with a lack of really good player alternatives for such a long period."
Wigan improved in the second half against the Black Cats, with Roger Espinoza and Angelo Henriquez impressing on their Premier League debuts.
Henriquez netted what Martinez expects to be the first of many goals in English football 11 minutes from time when the on-loan Manchester United man nodded in Shaun Maloney's cross.
"Whether he is scoring or not, Angelo has the profile of a goalscorer," added Martinez. "His movement off the last man is fantastic and he has that knack of being in the right place.
"Scoring on his Premier League debut is going to do a world of good for his confidence but I don't think it will change his way of playing or the high esteem he holds himself in.
"I have been very impressed with him. He is a young man who plays like a very mature footballer and I am sure he will be an important player for us from now until the end of the season."