The World Cup-winning French striker, 34, who had been sidelined for four weeks with what the club described as a muscular injury, missed two second-half chances to equalise during the 2-1 home defeat at the hands of Diego Maradona's side.
Vieira substituted his summer signing, who played for Hercules in Spain last season, after 73 minutes when the veteran looked to have run out of energy.
"David has been injured for a long time, as you know. We were forced to play him against Al Wasl, with his co-operation, as he made himself available for the match," said the coach. "It is not easy for a professional player to come back right away after being out for four or five weeks. We are talking about a lack of fitness and, for me, Trezeguet did his best against Al Wasl.
"He did relatively well, especially in his link-up play with the team, and he had some nice passes, some nice one-twos. We all know the quality of Trezeguet, his intelligence and knowledge of football."
Vieira said that, had the former Monaco and Juventus player been fully match-fit, he would have scored at least one his chances.
"He had two good chances to equalise for us, but couldn't score. I think that was down to his fitness more than anything," Vieira said.
"The presence of Trezeguet inside the box did give the Al Wasl defenders a lot to think about, even if he was not ready. All we need now is to get him moving a little better."
Trezeguet, with 171 goals, is the fourth-highest scorer in Juventus history and the best placed non-Italian in that list.
He was replaced on Saturday with another new face, Tony, who arrived from Al Jazira this month.
The Ivorian forward, who played for Al Wahda earlier in his career and spent last season on loan to Al Arabi of Qatar, is also lacking match fitness, and it may be a few weeks before Baniyas fans get to see Tony, Trezeguet and Andre Senghor, last season's Pro League top scorer, in the same starting line-up.
"Toni is also not ready, but right now both players [he and Trezeguet] could maybe play 45 minutes, or one could play for an hour and then be replaced by the other," Vieira said.
"Toni did not train with Al Jazira for the last 10 days of his time there."