Spurs took the lead in last weekend's North London derby but Emmanuel Adebayor was then shown a straight red card and the Gunners came back to triumph 5-2.
A goalless draw with Lazio in the Europa League helped Tottenham move on from the painful defeat and Defoe is now looking ahead to Sunday's clash with the Hammers.
He said: "Lazio was a decent point. When you come to grounds like the Olympic Stadium it's always difficult. The pitch wasn't the best."
"All the lads just want to forget about the Arsenal game now. I know it's easy for me to say this now but I honestly think that if Ade hadn't got sent off it would have been totally different."
Defoe spent time with West Ham at the start of his career and has been impressed by the way they have played this season, but he feels it is vital for Spurs to claim victory.
He said: "The way West Ham are playing is effective, so it's going to be a difficult game. Andy Carroll has been brilliant - he has worked hard and deserved a goal.
"West Ham work hard, they're honest and the way they play is effective. It's a must-win game for us and we need the three points then hopefully after that we can go on a good run."