Richards came on for Ivan Toney in the 79th minute of the 1-1 draw at the Kassam Stadium, having been out since early March with an Achilles injury.
Toney had netted the Cobblers goal in the first half, only for substitute Alex MacDonald to level things up in injury time.
But the return of Richards at least provided a big positive on a night that should have ended with three points for the Cobblers.
“Marc’s just been an unbelievable miss,” said Wilder of his 17-goal top scorer. “He’s been our captain and leading goalscorer so to get him out there is a great boost for us.
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“I thought the two young kids at the top of the pitch, Diego (De Girolamo) and Ivan, did a great job, they just ran out of legs, which young boys do.
“It’s Ivan’s first season as a full-time player and he scored a great goal and was a menace at the top of the pitch.
“He just ran out of steam a little bit and so did Diego, who has been injured.”