Pistorius was freed from prison in the South African capital Pretoria last October after serving one year of a five-year term for culpable homicide -- the equivalent of manslaughter.
But an appeals court upgraded the conviction to murder, which has a minimum standard sentence of 15 years. Due to his time already spent in jail and mitigating factors such as his disability, he may be given a lesser term.
Pistorius, 29, shot Steenkamp to death in the early hours of Valentine's Day in 2013, claiming he mistook her for a burglar when he fired four times through the door of his bedroom toilet.
Pistorius, sobbing heavily, hobbled on his stumps across the courtroom to demonstrate his physical vulnerability as his lawyers argued he should not return to jail on account of his anxiety disorder and depression.
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"I don't think he will get the 15 years," Ulrich Roux, who is unaffiliated with the case but has followed it closely, told AFP.
"His personal circumstances and the disability will be taken into account. One possibility is that part of the sentence may be suspended."
Nel also criticised Pistorius for filming a television interview, despite claiming to be too unwell to give evidence in court.
In the interview -- his first since the killing -- Pistorius said that he believed Steenkamp would want him to devote his life to charity rather than return to prison.
"If I was afforded the opportunity of redemption, I would like to help the less fortunate," he said.
"I would like to believe that if Reeva could look down upon me that she would want me to live that life."
Pistorius, who pleaded not guilty at his trial in 2014, has always denied killing Steenkamp in a rage, saying he was trying to protect her.
The sentencing may not be the end of the saga that has gripped South Africa and the world, as Pistorius or the state could launch a final round of appeals.
If jailed on Wednesday, he would likely return to the hospital section of Kgosi Mampuru II Prison in the capital Pretoria, separated from the general population of inmates.