The Red Devils sold star performer Cristiano Ronaldo to Real Madrid for £80million and have since brought in Antonio Valencia and Michael Owen to help compensate for the Portuguese's departure.
Liverpool, meanwhile, parted company with Xabi Alonso, also to Real, for around £25million and have signed Alberto Aquilani to help fill the void left by the Spanish midfielder.
Wenger has also sold personnel, having seen Kolo Toure and Emmanuel Adebayor move to Manchester City, but admits he is intrigued to discover how the Gunners' rivals cope this campaign.
"The question this season is who will cope the best with what has happened during this transfer market. Let's make sure it is us," he told the club's official magazine.
Fantastic"The big task for United is replacing the goals that Cristiano Ronaldo scored over the last few seasons.
"You can say whatever you like about Ronaldo, but you have to admit he had two fantastic qualities.
"Firstly, he was never injured. Secondly, he put the ball in the back of the net.
"I'm surprised that they didn't reinvest more of the money they got for Ronaldo, but they may do that in January.
"Manchester United, of course, will continue to be a major force in the championship, but you can see that where we drew four times 0-0, they won them 1-0, sometimes because Ronaldo made the difference.
"Michael Owen is worth a gamble I think, because he can be an impact player. Also you would hope that he can put his injury problems behind him at some stage.
"I don't believe he can ever be back to his complete best, but even if he gets to 95 per cent of what he was before, he will be a fantastic player."
AncelottiOf Liverpool, Wenger said: "Alberto Aquilani will be a good player [for Liverpool], but they have lost a great player in Alonso.
"He was the brain of the game for Liverpool last season. I don't expect them to struggle this season, but people might have a little question mark about how they replace Alonso."
Wenger also believes that Chelsea, now under the management of Carlo Ancelotti, will be a force this term, with the Blues having won both of their opening games of the season.
"I still think that Chelsea will be a main threat for the championship, but their squad is a year older which might have an impact when they have to repeat the games in short succession," he said.