Sabalenka needed treatment late in her meeting with Cirstea on Saturday before she fell 2-6, 6-3, 7-5 for her second defeat in three matches after a quarter-final loss to Hailey Baptiste at the Madrid Open last month.
"I feel like I didn't play well from the beginning till the end. I started really well, but I dropped the level," four-time Grand Slam champion Sabalenka said in Rome.
"I felt like my body was limiting me from performing at the highest level. She stepped in and played incredible tennis. She didn't really give me many opportunities.
"That was a tough one. But I guess we never lose; we only learn, so it's okay."
Sabalenka's disappointing claycourt swing follows a stellar run on hardcourts, during which she captured the Brisbane title, reached the Australian Open final and claimed back-to-back trophies in Indian Wells and Miami.
The Belarusian said she now hoped to recover her fitness in time for the French Open, which gets underway in Paris on May 24th, after losing last year's final against Coco Gauff.
"I'd say that probably it's like my lower back, connected to the hip, which limited my full rotation," the 28-year-old added.
"I guess we're just going to have some days off. We're going to spend it on recovery. That's the plan for now."