The former Rangers and Dunfermline defender suffered an ankle knock in the close season and is yet to feature for Thistle this term.
However the 24-year-old played for the Under-20s in a win over Inverness on Tuesday night and claims his injury is "improving all the time".
McMillan said: "Being able to get some match practice was good. Now I need to work on my match fitness and get myself back into contention for the first team. But the way the boys are going at the moment that won't be easy."
Alan Archibald's men have racked up two wins and two draws from their five SPFL Premiership matches so far.