Donaldson looks set for an extended lay-off after suffering a serious facial injury in the 1-1 draw in Dingwall.
County had already made three substitutions when the centre-back went off and Kettlewell and Steven Ferguson decided to keep the rest of the team as they were as they trailed to Stevie May's first-half opener.
Saints had late chances to put the game out of sight but the decision paid off when Billy McKay converted Josh Mullin's driven cross from six yards in the fourth minute of stoppage time.
"Firstly I need to say we hope big Coll Donaldson is OK," Kettlewell told County's Twitter account. "It looks a real nasty one, it looks like there may be a fracture of his cheek or something like that. So we wish him all the best.
"But obviously that changes your plans of trying to get back in the game. We had to shift to a back three and keep our four midfield and two strikers to try and have a go at it. The players dealt with that adversity really, really well.
"We felt that we might have something with Josh Mullin down the right-hand side with his crosses into the box. You always know you might have something with Billy McKay getting in between the frame of the goal. Fortunately enough it paid off in the end.
"We have to make sure in the next 11 games that it is a valuable point. We believe with the character and passion the players showed in the second half to haul themselves back in the game that it gives us a foundation and a chance."
All three Ladbrokes Premiership meetings between the teams have ended in draws after County defeated Saints in the Betfred Cup and Perth midfielder David Wotherspoon cannot believe his side have not beaten their opponents again.
"It was a great performance," Wotherspoon told Saints TV. "They didn't really create much and we created loads and should have been well out of sight. But we didn't take our chances and they got a luck deflection for their goal.
"We are gutted because it would have been a massive three points. It felt so comfortable throughout the game and it was a sucker-punch at the end.
"Even the first game of the season in the cup when we lost 2-1 I felt we dominated. I feel we have dominated every game but still haven't got three points off them. They could all count at the end of the season."