The Australian Diamonds skipper suffered a serious Achilles tendon injury in the Vixens' loss to the Queensland Firebirds on Thursday night and underwent surgery the next morning.
Vixens coach Julie Hoornweg said the team had been through an emotional journey after the loss of McMahon.
"We decided we are not a one-person team and that, as much as we love Sharelle and we were doing it for her - there was a lot of emotion in that today - we were doing it because of her," she said. "She's a great player and she's taught our young kids a great deal of skills."
The Steel battled to penetrate the defensive prowess of Vixens trio Geva Mentor, Bianca Chatfield and Julie Corletto, who swooped on any trickle of possession - too much of which was given away by a Steel team plagued by unforced errors and missed shots.
Halfway through the opening spell, the Southerners were struggling to find cohesion on attack and already staring at a 10-2 deficit after the Vixens unleashed a seven-goal spree.
Steel recovered slightly to narrow the gap, with defenders Leana de Bruin and Te Huinga Reo Selby-Rickit doing a solid job against the Vixens' new shooting combination of Kate Beveridge and Tegan Caldwell.
In a rare move, coach Robyn Broughton thrust goal shoot Daneka Wipiiti into goal attack, while Paula Griffin moved to goal shoot. However, the new arrangement lasted just a few minutes.
Down 12-7, the Steel initially found life didn't get any easier in the second quarter as the Vixens stormed to a 10-goal advantage. The Steel came back with seven unanswered goals to put it back in the hunt 20-17 at halftime and it quickly drew level 20-20, but couldn't punch through to take the lead.
In contrast, the Vixens contested every pass and were rewarded as Caldwell and Beveridge delivered 95% and 88% respectively under the post.
Steel captain Wendy Frew was notably disappointed.
"We just did silly things in the first quarter and it's hard playing catch up against a quality team like that," she said.
While the Steel's ANZ Championship title hopes were now dashed, Frew was adamant the team would continue to fight for victory in the last four rounds.
Its next assignment is against the Canterbury Tactix, in Dunedin on Saturday.
• Melbourne Vixens 42 (Kate Beveridge 23/26, Tegan Caldwell 19/20), Southern Steel 33 (Daneka Wipiiti 16/22, Paula Griffin 17/28). Halftime: 20-17.
