McGrath - along with Hampshire's new signing Shane Warne - will miss two phases of the season when he is required for one-day tournaments against South Africa in April and August.
But Worcestershire secretary Mike Vockins said: "We feel that Glenn McGrath for 80% of the time is more important than a lesser player for all of the season."
Worcestershire would then hope to have him back for the last month of the season. Vockins said: "I spoke to his manager (Warren Craig) yesterday and he was talking about Glenn flying back for our next game against Warwickshire on August 20.
"We are anticipating it will only be a short visit - very much like when we lost Tom Moody in August this year because of a tournament in Sri Lanka."
Earlier McGrath admitted that, despite his county commitments, his first priority is to his country.
Top priority
"My main priority is always the Australian team. They know that and I'm sure the Australian board and selctors know that," he said.
"The contract's for next season. I don't think it's for a specific time. It could cut a month and a half or two months off but then I'm not sure how much cricket we would have played at the beginning of the season."
Both McGrath and Warne were expected to be free after Australia finished their tour of New Zealand on 15 April, but on Tuesday the ACB announced there will be three one-day internationals in South Africa in April and another three in Australia in August - as the English season reaches its climax.
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