The 25-year-old came close to joining Liverpool over the summer only for Crystal Palace to pull out of the deal at the last minute.
Guehi has since played every minute of their Premier League campaign so far but now looks set to leave the club for free at the end of the season.
Speaking on the Men in Blazers podcast, Parish appeared to have ruled out selling the England international in January, suggesting Palace are planning their whole season with him.
“We needed to keep Marc because the difference between us having a stellar season and having a relegation-threatened season is us winning five games or not,” he said.
“So the margins in the Premier League are very tight and you have to remember that Michael (Olise) has gone, we’re OK, Ebbs (Eberechi Eze) has gone, we’re OK, Wilfred (Zaha) went, we’re OK.
“There’s a limit to how much you can keep doing that before you break down the fabric of the team.”
When asked about Guehi’s cancelled deal with Liverpool, he added: “It was a very marginal call. Marc was great about it as a person.
“Liverpool made the offer. It was a massive club, he was obviously interested in going. But he was fine about it, staying, if that was the decision we made.
“Obviously it means all of the ball is in his court tremendously as a player and he’s got the choice of wherever he wants to go. It was a very difficult one but I think definitely the right one.”