However, Salah has take English football by storm since returning to the Premier League with Liverpool in 2017, and Mourinho said he's been impressed by the way Salah has developed in recent years, per Matthew Treadwell of Sky Sports.
"The Salah I knew at Chelsea was a project player. Now he is one of the best in the world. He has developed incredibly well since then on every level. He was a fast, fragile boy, and [now] he is a fast, strong man; physically amazing.
"He was not psychologically adapted to the Premier League when he came from a small club in Switzerland [Basel] to a big club in England. I played him at White Hart Lane and The Eithad, and it was too much for him. Now he plays anywhere, Barcelona, Madrid, and he's like, 'I am Mo Salah, and I am afraid of nobody.'
"Liverpool have many strengths, but he is a nuclear strength."
Mourinho was speaking ahead of his side's meeting with Salah's Liverpool on Sunday in the Premier League. United are sixth in the top flight, while the Reds can go back to the summit after Manchester City's win over Everton on Saturday.
If they are going to go all the way in the Premier League this season, you suspect Salah will play an enormous part. Football journalist Dave O'Connell summed up just how productive the forward has been since his arrival at Anfield:
At Chelsea, it was clear there were still areas Salah needed to improve, as his searing speed often wasn't matched by sharp decision-making in the final third.
However, his time in Italy refined him as a footballer, and working under Jurgen Klopp at Liverpool in a setup that's tailored towards getting the best out of him has taken him to another level.
The rise of Salah has also prompted some questions about Mourinho's management, and Bleacher Report's Rob Blanchette questioned in the summer whether he can manage a particular type of footballer:
Salah has yet to score against United in his short Liverpool career but has been in blistering form lately. After a stunning hat-trick at Bournemouth last weekend, he netted the winner for the Reds in their crunch UEFA Champions League game with Napoli on Tuesday.
Mourinho's team have competed well against Liverpool in recent years, and the Portuguese tends to have a plan to halt Salah. However, based on the recent fortunes of both teams and the way Salah is performing, that'll be an almighty ask on Sunday.