The 30-year-old, the club's first-choice keeper, had been playing on with the problem but a decision was made after their Johnstone's Paint Trophy defeat to Southend last week.
The full extent of the damage will be revealed when he undergoes surgery but the former Bristol Rovers man is set to be sidelined for at least 10 weeks.
"Ryan has had a problem that has been going on now for six to eight weeks," Wilder told BBC Radio Oxford.
"He's not trained as much as he should have and I've just seen a dip in him.
"It's not his fault as he is obviously carrying it and the boy to be fair was brave enough to want to bat through this period.
"But I just made a decision in the week. I just didn't think it was fair on him and I think there have been a couple of soft goals and I do believe he would have saved all three penalties that went through him on Tuesday if he was fit."
Wayne Brown, United's 35-year-old player-coach, started in the 2-1 win over Bradford on Saturday while the club also have young keeper Max Crocombe available.
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