The Black Stars' medical team are working around the clock to get the Spartak Moscow forward back on his feet before their opening game against the USA on June 16.
Waris, who spent the second half of last season on loan at Valenciennes, damaged his quadriceps during a 4-0 warm-up victory against South Korea in Miami on Tuesday and could be ruled out of this summer's tournament in Brazil.
"He had a knock on his left thigh. We initially tried to see if he could go back to play but we realised it was an injury that needed to be taken care of and managed well," team doctor Baba Adam told the Ghana FA's official website. "We did an MRI on the quadriceps and what we realised is that, there was a tear.
"We hope to get him on to the field again pretty soon. However, we are still monitoring him to the deadline when we will give our final statement on his state of affairs.
"The Fifa deadline is [June] 15, so we will monitor till [sic] that day and see how things turn up.
Ghana will also face Germany and Portugal in Group G.