Off contract in June, it's been suggested Guehi could be sold by Palace in January.
"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," Parish told Men in Blazers.
"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."
On blocking Guehi from joining Liverpool on deadline day this season, Parish also said: "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."