The former England centre-back left Arsenal for Portsmouth in 2006 after scoring in the Champions League final defeat by Barcelona.
Campbell had moved across North London from Tottenham Hotspur in 2001 on a free transfer and was at Arsenal for five years in which they won two league titles and three FA Cups including the double in 2002.
"I didn't imagine he would play as many games as he has (since his return)," Wenger told reporters on Tuesday.
"I thought he would play in one or two games maybe but he got those extra matches because he played well.
"Sol has been a big support for me and the squad. He is very important in the dressing room because he is highly focused and highly motivated," he added of the former Spurs captain.
Campbell is certain to get a hot reception when he returns to his former club for the North London derby on Wednesday.
"I must say, every time he has played he has turned in a very good performance," added Wenger. "It's one of the good signings I've made. I've no concerns about him. I can tell you he'll play tomorrow."
IMPRESSIVE FORM
Campbell was brought in as back-up for Thomas Vermaelen, Mikael Silvestre and William Gallas but his impressive form and a spate of injuries have seen the 35-year-old play regularly.
He will make his 10th appearance since returning to Arsenal when they visit Spurs trying to sustain a title challenge while the hosts bid to finish fourth and make the Champions League.
Spurs manager Harry Redknapp, who took Campbell to Portsmouth from Arsenal and saw him captain the team to FA Cup success in 2008, was surprised he had returned for a second spell at the club.
"I could never see him going back to Arsenal in a million years," he said. "When I took him he wanted out and they wanted him out as well. He walked out on a fortune at Arsenal.
"It cost him millions and millions. He was losing about 3 million pounds a year (leaving Arsenal) but he just felt he didn't want to be there."
After three seasons at Portsmouth, Campbell had a brief spell at League Two Notts County under Sven-Goran Eriksson and then began training back at Arsenal.
Wenger had never re-signed a player during his 13 years in charge at the club when he brought Campbell back on a short-term deal but his renewed faith in the defender has paid dividends.
"It was a great lift for him to go back to Arsenal," said Redknapp. "For Arsenal to take him back was amazing."
Redknapp said Campbell would still be given a rough time by Spurs fans on Wednesday.
"I can't think of anyone who gets much worse (treatment) than him (in England)," he said. "They're not going to forgive him, the fans."
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