Chris Basham enters Saturday's game against Wigan Athletic one booking away from an automatic suspension.
The defender has collected four yellow cards so far this season, more than any other member of Chris Wilder's squad, and will be ruled-out of the visit to Nottingham Forest if he receives another during this weekend's fixture.
With the Steel City derby now only three matches away, Basham is United player most at risk of missing next month's clash with Sheffield Wednesday.
But four of his team mates, including fellow centre-half Jack O'Connell and centre-forward David McGoldrick, are edging closer to the cut-off point.
Together with Enda Stevens and Kieron Freeman, they have accumulated three apiece; meaning a caution against Paul Cook's side, coupled with another at the City Ground, would leave them ineligible for selection against Wednesday.
Oliver Norwood, the United midfielder, is also on three but a recent change to the Football Association's rule book means bookings are now viewed as "competition specific" and one of his yellow cards came on international duty with Northern Ireland.
Although Wilder has previously stated he does not ask players to alter their approach in order to avoid a ban, he is likely to remind Basham and his colleagues about their responsibilities.
"It's not something I go on about," the United manager said. "They (bookings) are becoming increasingly unavoidable in the modern game, especially in certain positions.
"But there are still ones that you can not pick up if you're careful. Everyone knows that."
Wilder, who was referring to those awarded for dissent, simulation or over-zealous celebrations, added: "Our lads know how to behave themselves. I think they've shown that on the pitch."
With Basham and O'Connell walking a disciplinary tightrope, Martin Cranie's return to fitness could prove particularly timely. The 32-year-old was forced to undergo a condensed 'pre-season' programme after joining United before August's transfer deadline having left Middlesbrough at the end of last term.
It could also explain why Wilder was keen to give Richard Stearman some game time during United's draw with Stoke City last night.
"Martin being available gives us another option," Wilder said. "It's good to have those, you can't have too many."