The men walked into the store in the 3200 block of South State Street just after 2:30 a.m. and ordered the clerk to open the cash register, police said, citing early reports.
One of the robbers cut the 47-year-old clerk on his lower right leg before both robbers fled with cash, police said.
Early reports indicated the victim claimed the robber was armed with a machete, but police would only say the robber had a "large knife."
The clerk was taken to nearby Mercy Hospital and Medical Center.
Police couldn't say whether the robbery was recorded by store surveillance cameras.