The 26-year-old will now undergo a course of medication before she can return to playing, a statement from the club said.
"It's a difficult one for Rhema but there's light at the end of the tunnel,” said Blades head coach Neil Redfearn.
“Her health is the most important thing and it's been caught early, the medical staff have done brilliantly and she's on with her medication now.
"We know we can rehabilitate her sooner rather than later and that's important for both her and us."
Lord-Mears played in all but one of Sheffield United’s 25 league and cup games last season and was awarded the manager's player of the season at the end of the campaign, which saw the club finish fourth in the Women’s Championship.
She was also named the club’s player of the year at The Star’s 2021 Football Awards.
The former Blackburn midfielder had started all three of the team's games this season, assisting the team's third goal at Coventry United before being sidelined ahead of Sunday's 3-0 win at home to Watford, which moved the Blades to the top of the league.
Redfearn added: "It's disappointing for Rhema. I've had the pleasure of working with her now for just over a season and I think she's an unbelievable talent.
"She's been a mainstay in that midfield with Maddy (Cusack) and Kasia (Lipka) and it looks different when she's not there.”
United Women travel to London to take on Crystal Palace on Sunday (2pm kick-off).