The Arsenal boss thinks the 25 year old is a promising player, but has indicated he was keen to secure the experience his squad has been criticised for lacking.
"I don't want to compare. Personally, I like Cahill as a player," said Wenger.
"Both are good players. Mertesacker is a good player. He is more experienced. He has more international experience.
"Cahill is a young, promising English player. I suppose he would have done well for us."