Oxford manager Chris Wilder wants Jon-Paul Pittman to prove he is worthy of a starting spot at the U's.
A summer signing from Wycombe, Pittman has struggled to get into the United line-up since his arrival and a substitute appearance at Crewe on Saturday brought ironic cheers from the travelling Oxford crowd.
But Wilder insists the attacker has yet to stake a significant claim to a place in his first XI.
"People will be wondering why he hasn't started or played more minutes - but he ain't done enough. That's the top and bottom of it," he told the Oxford Mail.
"Whether it's a reserve game or training, we've spoken about this to him and he knows.
"We've got a good relationship, but he needs to do more to get himself in the team."