Roma are struggling to meet the higher expectations produced in Mourinho's first term when the Giallorossi won the inaugural edition of the Europa Conference League.
Lazio, meanwhile, are on the upswing as Sarri's squad fine-tunes his intricate passing schemes.
The difference was clear in the Rome derby on Sunday when Sarri's Lazio edged Mourinho's Roma 1-0 with an early goal from Felipe Anderson to leapfrog their city rivals and move up to third place.
"This is one of the most intense derbies in the world. In London there are lot of derbies but there's no comparison," Sarri, who coached Chelsea in 2018-19, said.
"It turned into a tough, dirty match and their physicality made it difficult ... but we remained united."
Lazio have now kept clean sheets in six of their last seven matches and also have among the most productive attacks in the league. Roma, missing Argentina playmaker Paulo Dybala, have been kept scoreless in the league in two of their last three matches - both defeats.
"We're lacking lucidness in the final 25 meters," Mourinho said.
"We're missing that spark that Paulo provides."
Juventus beat Inter Milan 2-0 in the derby d'Italia for their fourth straight league win to move level with Roma, two points behind Lazio and Atalanta.
Filip Kostic assisted on both goals as Adrien Rabiot and Nicolo Fagioli each scored in the second half for the Bianconeri.
Inter are seventh, one point behind Juventus and Roma.
Earlier, Monza were boosted by the presence of Pablo Mari, the Spanish defender who was stabbed in the back in a knife attack at a shopping centre on October 27. Unable to play for a few months, Mari saluted the crowd and teammates before watching a 2-0 win over Hellas Verona.
Elsewhere, Fiorentina won 2-0 at Sampdoria for their fourth straight win in all competitions and Bologna beat Torino 2-1.
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