Lions' trio to miss England tour of New Zealand
Alex Corbisiero (C) of British and Irish Lions dives for a try against Australian Wallabies, at ANZ Stadium in Sydney, on July 6, 2013 - by Saeed Khan
Croft and Corbisiero have recently returned to full fitness but are not yet considered ready for what is set to be a gruelling three-Test series against the world champion All Blacks.
Meanwhile Leicester hooker Youngs will miss the June campaign to help support his wife, who is having treatment for an illness.
"Our thoughts are with Tom and Tiffany at this time and it is entirely right that he stays at home to support her and their daughter Maisie," England coach Stuart Lancaster said Wednesday.
"Everyone in the England squad sends them our best wishes," he added.
Youngs's absence added to England's hooker problems, with Dylan Hartley, David Paice and Rob Webber all battling for full fitness.
Harlequins' Dave Ward and Exeter's Luke Cowan-Dickie, both uncapped, could now be pitched into action for the first Test in Auckland on June 7.
In the backs, wings Christian Wade and Jack Nowell will also definitely miss the New Zealand tour through injury, while doubts remain over the fitness of Billy Twelvetrees, the Gloucester centre.
Twelvetrees suffered an ankle injury in Gloucester's 38-30 Premiership victory over London Irish.
Northampton prop Corbisiero has been out of action for seven months with persistent knee trouble while Leicester flanker Croft has returned after tearing knee ligaments on the season's opening day.
However, both players could now feature for their respective East Midlands clubs during the Premiership play-offs.
But just over a year out from the start of the 2015 World Cup in England, Lancaster does not believe the duo are yet ready for the rigours of a series with the All Blacks.
"Alex and Tom have worked really hard to come back before the end of the season but they will not have played enough rugby for them to be considered for such a tour," said Lancaster.
"It's the same with Christian Wade.
"They have all had significant injuries; it will be a short-term loss but a long-term gain for us and their clubs as we enter a huge 18 months.
"Jack (Nowell) needs to get his knee sorted so he will be ready for next season and the World Cup and it's the right decision for him to have the operation next week."
Lancaster will wait on the result of club play-offs before finalising his 40-man squad for the All Blacks.
None of the players taking part in the Premiership final will be available for the first Test in Auckland on June 7, which takes place just seven days after the Twickenham showpiece.