Lancashire and Middlesex are looking to replace Peter Siddle and Adam Voges respectively after the pair were called into Australia's Ashes squad.
Fast bowler Siddle, 30, had agreed to join the Red Rose from the beginning of the season until the end of July.
But he is set to miss most of the County Championship season, along with 35-year-old batsman Voges.
"The hunt for a replacement for Peter begins now," said Lancashire's cricket director Ashley Giles.
"When you sign an international bowler of his calibre there is always a possibility that this will happen, but we are still hopeful that Peter will be available for the start of our County Championship campaign.
"Although it is pretty frustrating for us, we are delighted for Peter."
|BBC Radio Lancashire's Scott Read|
|"Lancashire were expecting Siddle to miss out on selection and be free to play in the opening 12 Championship games. After an impressive end to his domestic season in Australia, he's caught the eye of the selectors."|
Meanwhile, Middlesex managing director of cricket Angus Fraser told BBC London 94.9. "It's a mixture [of feelings] really; delighted for Adam, but for us it's a blow.
"We've sat down thinking about contingencies and now is the time to start putting them into action.
"We've spent time considering possible replacements and now is the time to make the contacts and start sorting out what we're going to do once Adam leaves in the middle of May."
Voges was voted the Sheffield Shield Player of the Year in Australia during the winter, having made 1,358 runs at an average of 104.46.
In November, he was signed by Middlesex to replace captain Chris Rogers because of his fellow Australian's international commitments.
He fulfilled the same role in 2013, proving to be a success, and Fraser is hopeful the club will bring in someone capable of making a similar impact in place of Voges.
Fraser said: "It's interesting that when we signed Adam last year there were a few people questioning whether he was as big a name as we wanted. It's amazing how things change.
"I'm sure we'll get a suitable replacement in the next few days. You can't worry about it as such - you've got to get off your backside and hopefully the bloke you get in will do just as well as Adam would have."
Siddle lost his place in Australia's Test side towards the end of 2014, but played in the Big Bash Twenty20 tournament for Melbourne Renegades.
He spent time with Nottinghamshire last year, taking 37 wickets in 11 Championship games at an average of 31.48.