The Merseyside club have conceded 10 goals in their first three Premier League matches; six of which came in a humiliating home defeat against Chelsea last time out.
Martinez lamented his side's defending after they were hammered 6-3 by Jose Mourinho's title favourites at Goodison Park prior to the international break, a defeat that came after 2-2 draws against Leicester City and Arsenal.
The Spaniard is in no doubt as to where Everton need to improve when they go in search of their first top-flight win of the campaign at The Hawthorns on Saturday.
He said: "I thought we adapted well to each challenge in each of our first three games, but we have been slack in our defensive duties.
"That is something we need to correct. Our attacking play has been pleasing.
"It has been a surprise because we haven't lost any personnel, or any players from that back five.
"In the goalkeeper and the back four, we've got incredible experience - more than 1,100 Premier League appearances between them and that's quite a feat.
"We had 15 clean sheets last season, we've been very strong in the way we defend as a team and it's been one of those situations where everything that could have gone wrong in our box has gone wrong.
"But in the same way, we can't wait to get back the standards that we have defensively.
"We're quite aware of what we have to improve, but the margins are very small.
"It's not a need for a drastic revolution. We knew we were so good in that area that probably we relaxed a little bit and got punished."
West Brom were beaten 3-0 at Swansea City in their last Premier League clash, having also drawn their opening two matches.
Joleon Lescott could make his top-flight debut for Albion against his former club after recovering from a knee injury.
The centre-back came through a behind-closed-doors match against Hull City earlier this week and hopes to get the nod from head coach Alan Irvine.
"If the manager sees fit for me to be involved at the weekend in some capacity, then great," Lescott told the club's official website.
"Having played for Everton in the past, I've got a lot of close friends there, it's always a special game against them but it will be a tough one.
"I look at the fixtures throughout the season and, having played for Everton and Manchester City, they're the ones you're looking at."
Irvine – who left his post as academy manager at Everton to join West Brom in June - is waiting to learn the extent of a hamstring injury sustained by midfielder Youssouf Mulumbu on international duty with the Democratic Republic of Congo.
Ben Foster (thumb), Gareth McAuley (ankle) and Graham Dorrans (heel) are expected to be in contention after they were forced to withdraw from international duty, while left-back Sebastien Pocognoli (thigh) has also returned to training and it remains to be seen if Victor Anichebe (groin) will face his former club.
Everton will monitor the fitness of Leon Osman, Steven Pienaar and Samuel Eto'o (all knocks), but Ross Barkley, Bryan Oviedo and Arouna Kone are definitely sidelined with knee injuries.
