Shahzad could miss most of Yorkshire's 12 remaining group games – and possibly all of them – after he was called up by
England as cover for former Yorkshire star Ryan Sidebottom, who has a hamstring strain.
Shahzad joined the England squad at the National Performance Centre in Loughborough last night ahead of Saturday's one-day international against Scotland in Edinburgh, which is followed by five ODIs against Australia.
The 24-year-old must also have a strong chance of keeping his place for the subsequent three-match ODI series against Bangladesh, which finishes on July 12.
Yorkshire have three group games after July 12 but Shahzad's possible availability is unclear at this stage.
Shahzad will also miss Yorkshire's County Championship match against Lancashire at Old Trafford (June 28-July 1) and is likely to miss the Championship fixture at home to Warwickshire (July 5-8).
Yorkshire's problems are exacerbated by the absence of Tim Bresnan for the duration of the NatWest series along with that of Twenty20 overseas specialist Clint McKay, who is now unavailable until after Australia's final ODI against England on July 3.
Bresnan could be available for Sunday's Twenty20 game against Leicestershire at Grace Road, however, which England may use as a trial run to assess his fitness following a foot injury.
Yorkshire have added pace bowler Ben Sanderson to their squad for tomorrow's Twenty20 home game against Lancashire at Headingley Carnegie, when they will be looking to improve a record of three defeats in their opening four games.
West Indian pace bowler Tino Best comes into the side as a replacement for McKay.
Martyn Moxon, Yorkshire's director of professional cricket, said: "It's a blow to be without Shahzad, Bresnan and McKay for the foreseeable future, but we've just got to deal with it.
"It will be a challenge for us and hopefully we can step up to that challenge.
"It's always mixed emotions when your players get picked for England. On the one hand you're delighted for the lad concerned; on the other, it makes life more difficult for you.
"But it's great news for Ajmal, who deserves his chance.
"He's not an easy player to replace because he's got pace, penetration, he bowls well at the death, gets his yorkers in well and he can whack it a bit."
Sidebottom will remain with England to receive ongoing rehabilitation and further assessment.
Yorkshire members must book their seats in advance for tomorrow's game.
Members cannot gain entry to the ground in the normal way by using their membership swipe cards at the turnstiles.
Instead, those with FP T20 matches included in their membership package must obtain a ticket in advance with a designated seat number.
Tickets can be bought from the ticket office at the Indoor Cricket Centre on St Michael's Lane opposite the ground or by calling 0871 9711222.
Non-members' tickets are 20 adults, 10 concessions.
The match starts at 5.40pm and gates open at 3.40pm.