Both defenders were forced off by the 54th minute before Alfie Potter eventually broke the deadlock just after the hour-mark.
Peter Leven sealed it 10 minutes from time leaving Richardson to wonder what might have been.
"Both teams were feeling their way into it and testing each other out in the first half," he said.
"We felt quite confident going in at half-time at 0-0 and that we could push on in the second half and cause Oxford some problems.
"To lose the two centre-halves like that and having to shuffle personnel, and with the attacking threat that Oxford pose, made it very difficult for us.
"To lose Murphy and Winnard made it incredibly hard for us but I couldn't fault the lads' application and endeavour.
"But this was a big game for us and it's a bitter pill to swallow to be knocked out of the FA Cup when we were so close to going through in the first match."