The 18-year-old picked up the problem during a development XI match on Wednesday and left the ground in a knee brace.
Argyle are still awaiting results of the scan, but Sheridan knows the news will be bad.
"He's being assessed," he told the club's official website.
"I don't think it's a nice injury for the kid, which is a crying shame - he's worked his socks off and got himself fit.
"We'll have to wait and see. We're still waiting for the results of a scan. We'll keep our fingers crossed and do the best we can for him."