Former England captain Graham Gooch says Marcus Trescothick made the right decision to quit the Ashes tour if he was not in the right frame of mind.
Trescothick said eight days ago he "felt good"
Gooch told Five Live Sport: "I am shocked. He is a big player for England with a lot of experience.
"But he is not going to be much use if he is not mentally right.
"An Ashes tour is the toughest mentally to compete in because Australia don't hold back. So if he is not 100%, he is no use to them."
Gooch sympathised with Trescothick's plight, the 30-year-old having suffered a recurrence of a stress-related illness that also forced him early from India earlier this year.
"Playing top-level sport can be stressful, and stress affects people in different ways," Gooch added.
Marcus is a world-class performer and the Aussies would have been worried about him
"I can sympathise with him, having been through that scenario of being away from your family for long periods on tour.
"I personally think in 12 months Alastair Cook would have been opening for England anyway, but it is still a big loss.
"They have a very short warm-up period before the Ashes and would have already decided what the team was going to be.
"But at least they have got their best six batsman out there already. Whoever flies out is going to be carrying the drinks."
Former England spinner Phil Tufnell also questioned the timing of Trescothick's decision so close to the first Test.
"It's not great for England's chances. It's just a big blow when you didn't want it," Tufnell told Five Live.
"Marcus is a world-class performer and the Aussies would have been worried about him. If he gets in, he can score big runs and he's a good player against Shane Warne.
He is still young and hopefully he will come back and play for England again
"It's crucial for England to have a settled and solid opening partnership this winter and that's where you get the whole base from and the confidence grows."
Trescothick's Somerset team-mate Andy Caddick, the former England paceman, said the news was a "bolt from the blue".
"I remember talking to Duncan Fletcher about him when he was on the verge of playing for England and he was a very strong, mentally tough individual," Caddick told BBC News 24.
"Unfortunately this illness has been a bit of a bane for him for the last 12 months. It is just sad that a player of his quality has suffered from it again.
"But he is still young and hopefully with the support of his family and team-mates, he will come back and play for England again."