The Dane came on as a second half substitute and scored the first goal in the Gunners' 2-0 win at the Emirates but picked up an ankle injury when challenged by Cardiff keeper David Marshall as he slotted the ball home.
And Wenger confirmed he will miss Saturday's FA Cup tie with Tottenham.
"It's an ankle sprain and it doesn't look too good in fairness. For Saturday he is definitely out already but it is too early to say how long he will be out for."
Wenger was asked whether his injury - and the absence of Oliver Giroud - would mean a dip into the transfer market but the Arsenal boss responded: "Let's not go too early on the transfer front. We will assess Giroud and let's hope we have good news on Bendtner as well."
One option for Wenger will be to deploy Walcott in his favoured central striking role with the England man finding the net against Cardiff for the fifth time in as many games. The Arsenal boss admitted he was delighted with the performance of the former Southampton star.
"It showed as well when a player has been out for a long, long while and he comes back he can have a huge impact - it makes him very dangerous."
Walcott was on hand to convert and kill off a brave challenge by the Bluebirds by adding a second goal late on and Wenger believes his side showed a welcome resilience to keep plugging away to finally gain the three points.
"We were rewarded for our intelligence because we didn't concede a goal and that got us the three points we deserved. There is a special spirit in the side and we will fight until the last minute of the game. We created chance after chance in the last 20."
With Ole Gunnar Solskjaer - in the stands at the Emirates with Cardiff owner Vincent Tan - set for talks this week about succeeding Malky Mackay, the match could be caretaker-boss David Kerslake's last game in charge of the Bluebirds but Wenger felt the visitors put in a brave display.
"You've got to give credit to Cardiff - it was a fantastic effort today and if they can manage to repeat that they will stay in the league.
"He (Kerslake) has every reason to be proud. They have some good ingredients to stay in the league."