The teams drew 1-1 at Goodison Park last week and Allardyce is looking to guide his team to a first win in six matches.
West Ham took the lead in their last three games against West Bromwich Albion, Everton and Swansea City but were pegged back to 1-1 on each occasion.
"It's a concern, we are not as free scoring as we were," Allardyce told a news conference. "The results have dipped a bit but the performances haven't.
"One thing that has to happen is we get to more resilient defensively. We're becoming a much more creative side but our defensive weaknesses have been exploited too much.
"In order to win football games you need to make sure you're strong enough in both departments.
"We've only had four clean sheets so far and that's not good enough to keep you in the top six."
West Ham have slipped back to seventh in the Premier League but Everton are on an even worse run having lost four and drawn two of their last six matches.
They needed a last-minute equaliser from Romelu Lukaku to rescue a 1-1 draw against the Hammers at Goodison last week but gave an improved performance to draw 1-1 with Manchester City on Saturday.
Everton are 12th in the table but will take heart from a good recent record at Upton Park where they have lost just once in their last nine league and Cup games since 2002-03.
Allardyce missed Saturday's draw at Swansea with a chest infection but said he felt better after seeing Andy Carroll's stunning goal from the edge of the box.
"I had it live on a stream," he said. "It was a fantastic piece of skill and a brilliant finish from the time he first touched it to the shot."
(Reporting by Mike Collett, editing by Ed Osmond)
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