Jack Ross has challenged Chris Maguire to prove him wrong after almost leaving him out of the squad last weekend.
Maguire was due to drop out of the 18 to face AFC Wimbledon before Lynden Gooch withdrew hours before kick-off.
Gooch has since announced the birth of his baby boy and is back in contention for the trip to Oxford United.
Ross says Maguire has trained well this week and admits he has some tough decisions to make in the coming weeks.
"There's competition for places now," he said.
"Players need to be at it every single game and then in games they need to be at it.
"There's going to be a brutal reality to that to a degree.
"Players are sometimes going to have to suck it up.
"Chris has trained well this week.
"You have two choices as a player, you can go in the huff or you can say you want to prove people wrong," he added.
"I've never had a problem with players proving me wrong, whether it's a player I've let go or a player I've left out.
"I don't really have much of an ego that way. For anybody, not just Chris, who is in that position over the coming months ahead, hopefully will respond in the right way."
Sunderland are heading into a spell of five fixtures in a fortnight, and Ross has urged his players to seize the opportunity in the League One promotion race.
"What's helped us is having this little bit of a lull before it," he said.
"We'll need the squad I would imagine, you can't predict what will happen with injuries and illness.
"It's nice to go into a period like this with nearly a clean bill of health.
"We had a run earlier in the season but it was skewed because there were cup games involved.
"It's right that we haven't had that real explosive run either way.
"The draws have stilted us a little bit but not really hurt us.
"The upward trajectory has been steady rather than spectacular, but if we can get a spike then it will put us in a really good place.
"The players know it is a really important time for them."