A brain injury charity spokesperson says it was "incredibly dangerous" for Steve Smith to remain at the crease after suffering a blow to the head in the second Ashes test at Lord's over the weekend.
Smith had to leave the field after being hit on the neck near the rear base of his batting helmet by a fast, sharply rising delivery from Jofra Archer on Saturday.
Following tests before he was allowed to return at the fall of the subsequent wicket, and was eventually dismissed for 92.
A later revealed he had suffered a concussion and he missed the final day of the draw.
Luke Griggs, deputy chief executive for brain injury charity Headway, told the BBC that Smith was put in 'immediate danger' by returning to the pitch.
"All that time while he was out there, there has been all this brain activity and a concussion going on and it is exactly why he should have rested in that time and not put himself back in immediate danger," Griggs said.
"You have to be over-cautious when it comes to any type of concussion.
"The reaction time of a batsman facing a 90mph-plus delivery is incredibly small and yet it is absolutely vital they are fully concentrated.
"With concussion, the vision can be blurred and the brain can be slow at processing information. That leads to delayed reaction times and is just incredibly dangerous."
Cricket Australia has defended the team doctor who allowed Smith to resume his innings.
Alex Kountouris, Cricket Australia's manager of sports medicine, told a news conference in Australia on Monday he was "100%" satisfied with team doctor Richard Saw's treatment of Smith in London on the weekend, and he urged against any over-reaction to the sport's new concussion guidelines.
Smith complained of headaches later Saturday and was ruled out of the remainder of the match on Sunday after displaying concussion symptoms.
Marnus Labuschagne replaced Smith, becoming the first concussion substitute in international cricket, and scored 59 in an important innings that helped Australia draw the match at Lord's and retain a 1-0 series lead in the five-match series.
"The reality is only about one in five or six head impacts end up in concussion," said Kountouris, who added that Cricket Australia's research showed delayed concussion accounted for up to 30 percent of those cases. "If we pulled out every player who had a head impact, we'd be pulling out 80 per cent of players who don't have a concussion and taking them out of the game. So that would be an overreaction."
Kountouris said three other batters were hit in the head, but only Smith had a concussion.
"He didn't have a concussion at the time (Saturday)," he said. "If we took him out of the game, we would have been leaving him out of the game for no reason other than what we saw on the field.
"Our doctor is an expert in his field. He's trained to pick up even the minor signs of concussion. Everything he did was according to the protocol, he was very thorough and ... we're 100% happy with what happened over there."
The third test starts Thursday in Leeds, making Smith a doubtful starter.