The Algeria international had a very good game on Saturday - following his penalty miss against Liverpool at Anfield before the international break - as he scored his first goal at the Etihad with a sublime strike in the second half against Sean Dyche's team.
Fernandinho also found the net with a stunning strike and Guardiola hailed the performance of both players after the game.
On Mahrez, the City manager said as mcfc.com reported: “In Anfield, he played so good… that’s why he played today.
“He played with personality. Only the takers can miss a penalty. He scored a great goal, but the assists he created, how aggressive he was, was so good.
“Riyad made a good performance. We knew he was a talented player. He’s a guy who likes to play with the ball. He goes to the byline, can score with left or right.“He’s a good signing for us.”
And on the Brazil defensive midfielder, the Spaniard said: “Fernandinho in the period we are together, he is so important for us. No doubt about that.
“Today he was clever, his body shape, every time he received the ball he was ready for it. He was incredible – extraordinary.
“Today was one of the best performances this season. He is a key player for us.”