England beat France 34-6 at Wigan on Saturday to set up a last-four clash with the holders.
But Sinfield was unimpressed by his side’s quarter-final display and knows a repeat against the world champions at Wembley Stadium on Saturday would see them soundly beaten.
“I’m disappointed with our intensity,” he said. “We were a little bit off from what we’ve been in the group stages. Hopefully it’s a kick up the backside.
“Thankfully it wasn’t in the semi because we would have probably copped a heavy scoreline against us. We’ve a whole host of things to fix up and I’m sure we’ll be better next week. We have to be.”
Fiji last night completed the semi-final line-up with a 22-4 victory over Samoa at Warrington.
First-half tries from Aaron Groom and Waisala Naiqama helped Fiji build a 14-0 half-time lead.
And replacement Vitale Junior crossed with two minutes remaining to seal his side’s progress to the last four.
Fiji will now play Australia at Wembley on Saturday in a repeat of the 2008 semi-final.
THE FINAL FOUR
Rugby World Cup semi-finals
■ New Zealand v England. Saturday 23 November. Kick-off 1pm.
■ Australia v Fiji. Saturday 23 November. Kick-off 3.30pm.
Both matches to be played at Wembley Stadium