The 24-year-old was seen using crutches to leave the pitch, with Swans boss Carlos Carvalhal admitting that the problem had occurred before.
"I talk to the doctor, he has locked his knee in the past and it happened again today," the manager told reporters after the FA Cup stalemate. "Before it had stabilised and he had played and trained, this time it happened and it stayed locked.
"I spoke to him after the game. He says it feels much better, I don't think it is a big problem but let's see if there is a big problem or not a problem."
Mawson moved to Swansea from Barnsley in August 2016 for £5.5m.