Skipper-for-the-night Sam Dickinson and last season’s club captain Lee Dickson were both forced off inside the opening 25 minutes at Sixways.
But flanker Nutley and scrum-half Kessell made impressive impacts from the bench, and were eventually two of Saints’ better performers in an 18-17 win.
And Mallinder said: “I was really pleased with both of those.
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“You don’t want to make early replacements if you can help it, but Ben and Tom came on and played a very big part in our success.”
And when asked about the nature of the injuries sustained by Dickinson and Dickson, Mallinder said: “They’re both a little bit sore.
“We don’t know the extent of the soreness, but Sam Dickinson’s hurt his knee - he got his studs trapped in the surface.
“Lee Dickson took a blow to his ribs.”