Assistant manager Craig Shakespeare told BBC Radio Humberside: "The knee's not too badly swollen, which would point to not too long term.
"Until we get the scan results we're not quite sure, but we're optimistic he won't be out too long."
The 19-year-old signed permanently from Manchester United in January after impressing during a loan spell.
Stewart has made a total of 14 appearances for the Championship club, having failed to make a first-team outing at Old Trafford.
Source: BBC Sport
Source: BBC Sport