Demands to know why a violent robber with 100 previous offences who has absconded from an open prison was there in the first place have been made.
A judge said Phillips's attack was "catastrophic" for his victim
Andrew Clifford Phillips, 40, who was jailed for 11 years in 2005, has failed to return to Prescoed in Monmouthshire.
Conservative MP David Davies said it was a disgrace he was there so soon after sentencing.
But the Ministry of Justice said open prisons were to help offenders reintegrate into society on release.
Phillips was convicted of mugging a 50-year-old man in Cardiff Bay, who fell back onto the pavement and was left unable to walk.
The prosecution called it a "senseless attack" and the judge said it had a "catastrophic effect" on the victim.
Mr Davies, who represents Monmouth, said Phillips, from Pentwyn, Cardiff, should not have been moved to the open prison less than three years into an 11-year sentence.
He will write to the Justice Minister David Hanson to ask why Phillips was in an open prison.
"The normal process is that people aren't sent to open prisons until the end of their sentence," he said.
"Nobody should be going in there with more than 12 months to run. This man has served two years of an 11-year sentence for a violent crime.
"These prisons are no punishment. It's a disgrace."
Gwent Police has warned the public not to approach Phillips, who is not a sex offender, and said it was believed there may be a potential risk due to the fact that he is no longer in the custody of the prison service.
Jenny Willott, the Liberal Democrat MP for Cardiff Central, which includes Pentwyn, said that while open prisons can be a "valuable part of rehabilitation", she was "stunned" that Phillips had been moved so soon.
"It does seem very unusual that someone who has served so short a distance into a sentence should be moved to an open prison," she said.
A spokeswoman for the Ministry of Justice would not comment on individual cases but said: "To release prisoners directly from a closed prison without the resettlement benefits of the open estate would undoubtedly lead to higher levels of post-release re-offending."
She said the level of absconding was at a 10-year low per head of prison population and that prisoners moving through their sentence may be re-categorised and allocated to open conditions when their risk has been assessed as having reduced sufficiently.
Gwent Police say Phillips is white, medium build, 1.75m tall with cropped hair and has tattoos on his neck and forearm.
He was last seen wearing a blue Next t-shirt, blue Wrangler jeans and black and silver Nike trainers.
Anyone with information is asked to call Gwent Police on 01633 838111.