Algeria international Hachi made his debut last night soon after the club announced he had penned a four-year deal at Chloorkop following the expiry of his contract at ES Setif.
Hachi impressed at left-back at Dr Molemela Stadium in the absence of the injured Tebogo Langerman.
"Normally I'd not have started him, same as like Anele [Ngcongca], I did not start him, just for them to come for the first time for the match," said Mosimane.
"But the problem is that Langerman's got a calf injury strain and 'Muzi' Mashaba is injured again. Muzi, he has been away for five months and he came back and he's out again.
"So Fares it was the right timing hey... right, right time when I didn't have a left-back because, usually I use Samuel Julies, I use Siyanda Zwane. But I didn't want to use Samuel Julies because they've had big games in the last two games in the MDC.
"And he needs to concentrate there to try and win the title in the MDC. But... it's like he's been here for some time and he adjusted well."
Meanwhile, the CAF Coach of the year in 2016 said the experience of being champions of Africa shone through after defeating a stubborn Celtic via a solitary strike from Themba Zwane.
"We've got experience from the Champions League and we know how to play this kind of the game," he added.
"We managed to score the goal and it was difficult but let's be honest, they had chances. They had two chances and they could have scored but you know how football is, football is not about, 'we had chances and that', you must take it. But we were fortunate, we took a goal and we move on."
Downs, who rose to fifth on the log with 22 points from 10 games, will be minus suspended defender Wayne Arendse when they next host Orlando Pirates at Loftus Versveld Stadium on Saturday.