The 2011-12 winners start their European campaign this season with a home clash against Schalke on Wednesday, with Sporting Lisbon and Maribor the other two Group G teams.
Chelsea extended the only remaining 100 per cent Premier League record on Saturday by coming from behind to beat Swansea City 4-2 and make it four wins out of four.
And while Mourinho's men are undoubtedly the red-hot favourites to progress with ease in the Champions League, the former Porto, Inter and Real Madrid manager has sounded a note of caution.
"The group is more difficult than people think," said the Portuguese.
"It's not because I'm Portuguese, but the Portuguese teams are always very difficult.
"The Portuguese league is not a strong league, but the three top teams are very strong.
"You go through Chelsea's history and you'll see when Chelsea play Benfica it was difficult, when Chelsea play Porto it was difficult, and when Chelsea play Sporting in a couple of weeks it will be difficult.
"The Germans are the Germans and Schalke improved a lot in relation to last season.
"So without speaking about Maribor because I haven't analysed them yet because they are the last team that we play against, our group is hard.
"We also have fixtures which are like a poison gift for us. Every time we play in the Champions League, the next match we play is against Man City, against Arsenal, against Man United and against Liverpool.
"So we have difficult weeks ahead of us."
Chelsea reached the Champions League semi-finals last term in Mourinho's first season back at Stamford Bridge, before being beaten 3-1 on aggregate by Atletico Madrid.
