The Stamford Bridge club's captain has started in the wins over Hull City and Aston Villa at the start of the Premier League.
But former England international Terry is aware he cannot be certain to play every game amid competition for places with the likes of Gary Cahill and David Luiz.
"No guarantees you will be in the side, nothing," Terry told Sky Sports when speaking about Mourinho's return for a second spell in charge of Chelsea this summer.
"Whether you are captain or not, he made that very clear from day one."
Next up for Chelsea is Monday night's trip to title rivals Manchester United, where David Moyes will take charge of his first competitive game at Old Trafford.
"I don't think it is going to make or break the season," said Terry. "It could be one of them - a head-start for either side.
"But we have a good record going to Old Trafford over the last few years and that will serve us well.
"Obviously it is David Moyes' first game at the stadium so we are expecting quite a hostile atmosphere anyway."
Terry added: "But we will go there and look to stay solid for the first 10 or 15 minutes anyway. We know it is going to be a tough place to go and they have had a good win against Swansea.
"But the two wins for us will serve us well and we go there full of confidence."