The 19-year-old joined for the season and has scored once in 11 League One games as the U's have climbed to ninth.
But the Wales international fractured a metatarsal in the first half of the 2-2 draw and boss Karl Robinson said he will be out for "12 weeks or so".
"It's such bad luck for Ben," Robinson told the club website.
"He's been showing everyone what a good player he is. He is a great lad and will listen to the advice and he is determined to keep doing what he can and come back fitter and stronger than ever.