The Portuguese manager is no stranger to the Egyptian's talents having signed him for Chelsea from FC Basel during the 2014 January transfer window.
Salah lasted just under a year at Stamford Bridge before being shipped off on loan to Fiorentina. He made just 13 appearances for Chelsea during the second-half of the 2013/14 campaign and the first-half of the 2014/15 campaign.
The Egypt international spent the second-half of the 2014/15 campaign on loan with Fiorentina before joining Roma on a season-long loan at the start of the 2015/16 campaign. He joined the Italian capital club on a permanent basis the following season.
Salah impressed with Roma last season, scoring 19 goals and assisting 15 in 41 games across all competitions, which attracted the attention of Jurgen Klopp. Liverpool forked out over £35m ($42m) to bring the player to Anfield this summer.
The Egyptian winger has taken to the Premier League like the proverbial duck to water and impressed from the start, unlike his previous spell in England.
Salah has scored six goals in 11 games across all competitions so far in this campaign and Mourinho has warned his team about his quality ahead of their Saturday (14 October) lunchtime clash against their archrivals.
"I'm not surprised with Salah's performance this season. I was aware of his great capabilities when he was with me at Chelsea," Mourinho said, as quoted by the Manchester Evening News.
"He improved during his spell at Italy, became more mature and improved physically. Players need time to adapt to their new teams. Some of them remain and others prefer to move to another club. It is normal in football," the former Chelsea manager said.
There will be more pressure on Salah going into the game owing to Sadio Mane's injury. The Senegal forward suffered a hamstring injury while on international duty and is expected to be out for at least six weeks.
The Egyptian winger is in good form, though, having scored both his country's goals to help the team qualify for next summer's World Cup in Russia.