Oxford manager Pep Clotet has urged winger Marvin Johnson to show more consistency after Oxford's 4-3 EFL Cup defeat by Cheltenham Town on Tuesday.
The 26-year-old scored Oxford's first goal but was replaced after 67 minutes with his side leading 3-2 at the time.
Johnson has been linked with interest from Leicester City among other clubs.
"He's responsible for his own performance," Clotet said. "The players who go somewhere are the players who perform week in, week out."
Birmingham City and Aston Villa are also understood to be considering making offers for the former Motherwell player.
"Players who can perform in the Championship and Premier League always perform, this is the way forward," Clotet told BBC Radio Oxford.
"He tries to play well, you have better days and sometimes bad days. He wants to contribute to his team-mates."
Johnson scored six goals in 52 appearances for Oxford last season after signing a three-year contract. The U's paid Motherwell an undisclosed fee on transfer deadline day in August 2016.