Norwich man Lafferty sat out training on Wednesday because of a groin injury picked up on Tuesday.
NI boss Michael O'Neill has expressed confidence that top scorer Lafferty will be OK to face the Poles in Nice.
But Grigg, 24, said: "I am in the form of my life and the best shape I can possibly be."
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The Northern Ireland camp are waiting for the results of a scan on Lafferty who scored seven goals during the successful qualifying campaign.
If Lafferty isn't fit for Sunday's opener, O'Neill has several options up front besides Grigg.
Should the manager adopt his favoured 3-5-2 set-up, he could well plump for a Conor Washington and Jamie Ward pairing.
However, Grigg, 24, scorer of 28 goals last season as Wigan clinched promotion from League One, insists: "I will be ready if I am needed.
"I am sure Kyle will be fine, but if he is not I am prepared to step up.
"Kyle has been massive for us in our qualifying campaign and has proved his class.
"But it has been a long time since we have had four strikers on form and chipping in with goals."
Grigg, who has eight caps, scored his first international goal in the 27 May 3-0 home friendly win over Belarus.
He has played in three successive League One promotion-winning campaigns having previously been with MK Dons and Brentford.
"Since I signed for Wigan things could not have gone any better," said the Solihull-born striker.
"My place in the Northern Ireland squad was in question but since Christmas the goals have been flying in and I have helped my club win promotion.
"It was great when Michael O'Neill told me I was in. I felt I could not have done any more."
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Grigg's exploits with Wigan received extra attention when club fan Sean Kennedy reworded a pop song and put it on Youtube.
"Will Grigg's on fire" made it into the download charts and has raised money for charity.
"It would be great to hear the fans singing at Euro 2016 when I am on the pitch," he said.
"But first I have to give them something to sing about."
Wigan's Will Grigg says he is ready to step in if Kyle Lafferty is not fit for Northern Ireland's first Euro 2016 match against Poland on Sunday.