Mitchell initially joined Sale on a consultancy basis earlier this season, with a view to a permanent move.
The former New Zealand boss was still technically employed by the Golden Lions, though, and was suspended in June due to allegations of misconduct.
Having been cleared of those charges, the New Zealander then announced his intention to re-join the South African side and reportedly resumed work this week.
However, Diamond said: "John's negotiating his way out in South Africa and he's set to join us in the next 48 to 72 hours."
Diamond was speaking after the narrow triumph over Sarries, during which the hosts scored three tries through Mark Cueto, Fraser McKenzie and a penalty try.
"We were poor in the first half defensively and Saracens are obviously a good side," he added. "Even though they were missing quite a few players, they know how to win games.
"It looked like the game was slipping away from us at half-time. But fair play, we injected a bit of youth and enthusiasm, and we managed to get back into the game after the break."
Sarries forwards coach Paul Gustard was frustrated by the set-piece issues which blighted their performance.
Although they touched down twice through Kameli Ratuvou and Joel Tomkins, a late Rob Miller penalty condemned them to defeat.
Gustard said: "For us, it doesn't matter who was playing. We regard all these guys as first-team players, so it's not about the guys who are missing.
"In the first half, I thought our set-piece in terms of our scrum and maul defence was poor and compromised us in terms of field position.
"Going into half-time with the score as it was (20-12 up), considering the issues we had in that set-piece area, we were pretty pleased.
"I think Sale got away with a little bit in terms of how they set up their maul, but unfortunately we couldn't deal with it very well."
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