The 32-year-old former Manchester United player limped off during the first half of the weekend's 3-2 Peace Cup victory over FC Groningen in Korea with suspected knee ligament damage.
With summer signing
O'Neill told the Sunderland Echo: "Wes is the worry. It could be a bad one, but we really don't know at this stage.
"You are told one thing at half-time and another thing at full-time, so really, it's too early to tell.
"We will probably only find out once we are home and it settles down, but hopefully it won't be too bad because he's an important player for us."
Brown arrived at the Stadium of Light last summer from Old Trafford, where injuries had repeatedly hampered him, but started each of the first 20 games of the new campaign.
A groin strain suffered in a famous 1-0 Barclays Premier League victory over Manchester City on
Cuellar, 30, is less of a concern, and his premature departure was more precautionary.
O'Neill and his players were heading back to England after finishing third in the four-team tournament, with substitute
The Ulsterman will hope to step up his recruitment drive on his return - former Aston Villa defender Cuellar is his only new acquisition to date - with attacking players his priority.
Reports over the weekend suggested he had returned to Villa to snap up
He said: "Undoubtedly, what is focusing my mind more than anything else is the need to strengthen our attacking options.
"We have lost Nicklas Bendtner after his loan, we have lost
"What I am looking to do is bring in attacking players who have quality and who have more experience than we have now, because, when you look at our squad in terms of attackers, we are short on experience and it's something we need to try to address."
English Premier League
Chelsea striker Demba Ba would relish more competition with Fernando Torres next season - but is also keen to partner the Spaniard.