Vela, 27, will miss Saturday's League One clash against newly-promoted Cambridge United at Montgomery Waters Meadow with his media knee ligament injury.
The problem was sustained at the back end of September and has now seen the influential midfielder sidelined for four weeks, which was given as an initial timeframe following scans.
But the ex-Bolton man, who has already missed six games in all competitions, will now not return until Saturday week's league clash at Lincoln at the earliest.
Town will make a late call on an ankle injury to George Nurse, after the defender limped out late of last weekend's home win over MK Dons.
The knock forced Nurse out of Tuesday's defeat at Oxford, where boss Steve Cotterill said the injury was swollen.
Cotterill said: "Vels won't be available for the weekend, we knew when he did it that it was quite a serious one.
"We've just got to be patient a little bit, not that we want to be, but he's definitely heading in the right direction.
"There's another improvement in him today, we've just got to be very careful, we don't want him to do any more damage because then it's going to be longer term. We've got to be careful as much as we don't want to be at the moment."
The Town chief, whose side’s three league wins this term have all come at home, was not happy with the manner of Oxford’s first goal in the hosts’ 2-0 win at the Kassam Stadium on Tuesday night. The U’s broke the deadline less than a minute into the second half, straight from a cross.
Cotterill, however, wants to see his side improve in the final third by way of hitting the target.
“We’ve got to be better to stop shots being blocked and work the goalkeeper,” Cotterill added of his team’s fortunes. “We work on all of those, there isn’t an aspect of our play that we haven’t or don’t work on.
“Sometimes, you only need an individual to have an off-day in that one moment and then it can be costly for you, that’s what we’ve got to try to not let happen too many times in any particular phase of play.
“You might make one mistake, might make two, but you won’t get away with three or four, the lads know that.”
Cambridge have adapted well to life in the third tier for the first time in 19 years and sit 16th after 12 games.