Frenkie de Jong was sent off in the 65th minute at the Ramon de Carranza stadium but holding on for a draw will earn little credit for Koeman, who was also shown a red card in injury-time.
Whether Koeman is still in the post for Barca's game at home to Levante on Sunday remains to be seen but it appears now to be more a matter of when the club can find a replacement than if he will continue in charge.
"You have to always try to win each game, but you have to be realistic, you have to see the squad we have, the people we're missing," said Koeman.
Barcelona are already seven points behind LaLiga's early leaders Real Madrid, albeit having played a game fewer, to go with a poor start in the Champions League after last week's 3-0 opening defeat by Bayern Munich.
A 1-1 draw with Granada on Monday did little to boost morale, with Koeman coming in for heavy criticism after finishing the game with central defenders Gerard Pique and Ronald Araujo upfront.
Disappointment with the team's style and doubts around his future prompted a defiant reaction from Koeman in a press conference on Wednesday.
He refused to answer questions, instead of reading out a statement that asked for patience and insisted the process of rebuilding the team "deserved unconditional support".
That was taken as a clear message to club president Joan Laporta, who was reluctant to be drawn on Koeman's future before the match against Cadiz.
Laporta told reporters nothing would be decided on the basis of the result alone while admitting "if we have to take decisions at a certain time, we will take them".
After the match, Koeman said: "He was in the hotel and we said hello before the match. I'm sure there will be more days to talk if the club wants to talk."
Koeman started with 17-year-old Gavi in midfield and 18-year-old Yusuf Demir in the front three, while the much-maligned Luuk de Jong continued down the middle.
The first half was almost entirely forgettable, with neither team capable of creating a real chance and a weak shot from Memphis Depay the closest Barcelona came to a goal.
Alvaro Negredo's shot with the outside of his foot drew a good save from Marc-Andre ter Stegen early in the second period while Luuk de Jong curved a fantastic cross to the back post but Depay miskicked his finish.
Barcelona were gaining in confidence but two yellow cards from Frenkie de Jong in four minutes checked their momentum.
De Jong was booked for the first time in the 61st minute for throwing back an arm into Alberto Perea and then for a second time in the 65th, as a careless touch forced him to slide on the stretch, even he flicked the ball away from Alfonso Espino first.
VAR would surely have spared De Jong but it could not intervene over a second booking.
Cadiz sensed an opportunity and should have scored with 10 minutes left when Espino slipped in Salvi but Ter Stegen came to the rescue, blocking the finish with his legs.
The game turned wild in the final minutes as the ball flew from one end to the other.
Philippe Coutinho spurned a late free-kick, Salvi's close-range finish was blocked and as the clock ticked into the 94th minute, Depay fired just wide of the far post.
In the madness, Koeman was shown a red card as well, the Dutchman smiling at the final whistle with a face of disbelief.