The Gunners were leading the way in the early stages of the campaign, but are now 13 points behind first-placed Liverpool in the table.
"I have been surprised by Arsenal's dip this year," Allardyce told reporters. "I really fancied that they would go right to the end. We've seen a decline in Arsenal and an increase and uplift in Liverpool.
"Look at his injury list and you can see why - there is no doubt about that. Even though they have top players, the amount of players who have been missing recently has disrupted the squad and the flow of the team. The injuries have had an affect."
West Ham and Arsenal will go head to head at the Emirates this evening.