He has been recalled by Cricket Australia for treatment on a foot injury suffered while bowling during the County Championship loss to Sussex.
The county hope New Zealand's Mitchell Santner, who is playing in the T20 Blast, can be his overseas replacement.
Hastings, 31, has taken 16 wickets at 34.38 and scored 139 runs at 23.16 in his six Championship games this summer.
On Friday, he hit the second fastest Twenty20 half-century in Worcestershire's history off 19 balls against Birmingham Bears.
Director of cricket Steve Rhodes told BBC Hereford & Worcester: "Cricket Australia have asked for him to go back home.
"We thoroughly understand that - they want John Hastings back fit and want to monitor his rehab, so we'll take it on the chin.
"We'll certainly look at alternatives. In the past we've brought in Miguel Cummins, Kemar Roach and Shannon Gabriel but the West Indies are here this year.
"So we've just got the find the right one because we're desperate to go up."
Worcestershire are currently second in Division Two of the County Championship, 17 points clear of both Sussex and Kent, who have a game in hand.

The Stagecoach 585 service and the 4x4 crashed near the Rowan Tree restaurant, north of Ardrossan, at about 14:15.
The Scottish Ambulance Service has declared the scene a major incident. Two air ambulances attended, along with specialist trauma teams.
The road was closed while emergency crews dealt with the crash.
Police Scotland said: "At around 1415 hours on Monday 21 March, police and emergency services responded to reports of a serious road traffic collision between a service bus and a 4x4 vehicle on the A78, north of Ardrossan, close to the Rowan Tree Restaurant.
"There have been 12 people injured as a result of this incident. One person has been pronounced dead at the scene of the incident.
"Two people have been taken to the Queen Elizabeth University Hospital by air and by road ambulance. Nine others have been treated at the scene or transferred to Crosshouse Hospital for observations."
A spokesman for the ambulance service said: "There are 11 casualties, several of whom are in a critical condition."
A spokeswoman for Stagecoach confirmed that the 585 service from Ayr to Greenock had been "involved in a serious incident...on the A78 near Ardrossan".
She said: "Our first thoughts are with those affected by this incident.
"The safety of our passengers and staff is our highest priority and we are assisting the police fully with their investigation into the circumstances."
A spokesman for the Scottish Fire and Rescue Service said: "Our crews are at the scene of a serious road collision on the A78.
"We received a call from the ambulance service at 2.15pm today and firefighters reached the incident three minutes later.
"There were six appliances at the scene and firefighters worked alongside police and paramedics to assist those involved."
Worcestershire will be without Australia all-rounder John Hastings for the remainder of the 2017 season.
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One person has died and 11 others have been injured - two of them critically - after a bus and 4x4 vehicle collided on the A78 in North Ayrshire.