Oxford boss Karl Robinson was close to tears on the touchline as substitute Shandon Baptiste was stretchered off with a serious knee injury in the 1-0 defeat at Brentford.
Baptiste left Griffin Park to warm applause from the home fans after a suspected rupture of his cruciate ligament when he fell awkwardly in a challenge with Bees midfielder Josh McEachran.
Robinson absolved the former Chelsea man of any blame before bemoaning his side's growing injury list after Brentford's top scorer Neal Maupay settled the tie from the penalty spot.
"This was his (Baptiste) first game back from a bad injury after nearly three months out and I saw the pain and adversity he went through and how he came back the hard way, running his socks off," he said.
"There is a lot of movement in his knee and it doesn't look good but we'll pray that the scan brings us better news on Monday. I'm on the floor with him and was nearly in tears. He's in bits because we are people and being a good person is vital because results come and go just like praise and criticism.
"Brentford is a great club with a very talented team. I love the way they play and their patience, but my players were excellent.
"We have 11 of our squad on the treatment table and we are putting defenders on the wing and wingers up front. They gave me all they have and that's all you can ask for. We've been poor in recent weeks but today was excellent."
Robinson felt his side deserved a late penalty for a foul on Marcus Browne.
He added: "That is clutching at straws but the most important thing is that we showed a lot of desire which will give us something to take into the league. There's a long way to go but we were okay today."
Brentford head coach Thomas Frank called on his midfielders and wingers to chip in with goals to ease the pressure on 16-goal striker Maupay.
The little Frenchman won and scored the penalty that took his side into the fourth round, but Frank is keen for the rest of the side to share the burden.
"I would definitely prefer other players to be scoring from other positions as well, but the good signs are that both our wing backs had chances to get on the scoresheet today," he said.
Midfielders Ollie Watkins, Said Benrahma and Sergi Canos all toiled against a resolute Oxford defence, but failed to work the keeper like they were earlier in the season.
Frank said: "They are all very good young players and still learning and developing. They have proved what good players they are with goals and assists but they will always be a little bit up and down because of where they are in their careers."
Frank refused to rule a dip into the transfer market to boost the 'goals for' column in or out, adding: "We'd like to have two players for every position but we have Chiedoze Ogbene back from loan now so we will see what happens."
The Danish coach believes his side is now definitely through the bad spell that coincided with his appointment, and added: "We are a lot more settled now, have stabilised things and are capable of making some fantastic results."
