Henry has been training with Arsenal in the MLS off-season and is not required to return to the United States until March.
Wenger revealed it was his idea to bring back the 34-year-old, who scored 226 times in 370 appearances for the club before moving to Barcelona in June 2007.
But he has warned Arsenal fans not to expect the Henry of 10 years ago.
Wenger said: "You can never take away from people what they have done, that will remain forever. Nobody expects him to be what he was 10 years ago, we just want him to help out.
"I think it is just credit to him to be wanting to be helpful to a club that has given him so much. Therefore I feel there is no special pressure to be the player he was before.
"You cannot put too much pressure on him. He is here to help out, not to suddenly be the leader, the main player in the squad, and I respect his desire to help us get through a period where we lose two strikers.
"He and I are conscious that he is 34 years old, but he has not lost his class or his brain. That will be just enough for us."
Wenger wants Henry, who is available until the MLS season restarts in March, to fill the supporting striker void which will be left when Marouane Chamakh and Gervinho leave for the African Nations Cup next month, although Arsenal are trying to delay their departure by two days to leave them available for the FA Cup tie against Leeds on January 9.
The Gunners boss insists Henry needs two weeks to be fully match fit but in theory he could be available for seven Premier League matches, including key encounters against Manchester United and Tottenham as well as FA Cup ties and Arsenal's Champions League last-16 encounter against AC Milan.
On the prospect of Henry partnering the Premier League's leading scorer Van Persie, Wenger said: "When they play together we will do very well. They are two good players who can give the ball and score, so why should they not do well together?
"Robin van Persie is our main striker and it will remain like that.
"He (Henry) still has his pace, because he always had exceptional pace. What you lose with age is the capacity to repeat it.
"He still has class and quality. I don't worry about him at all. He is there to help, nothing else.
"When we need him he will come on and play. He wants to help out for two months, because this is a club he loves.
"He has exceptional talent, he is a very intelligent man and has a desire to help the club, so it can only be positive."
Wenger pointed to the success of former players Sol Campbell and Jens Lehmann, who also returned to the club in supporting roles.
"Our fans are happy that players who have done well for the club want to help out again," Wenger said. "That is very positive. It shows that Arsenal have a special link with the players because we had Sol Campbell and Jens Lehmann who helped out for a while and did very well. Now we have Thierry, it is a continuity of the special link with our former players."
Wenger, despite insisting the Henry deal was "certain to happen" did sound one note of caution.
He said: "The agreement and the insurance (still needs to be done), I don't take care of that, but that is not completed and we need to respect all the agreements we need with the MLS to finalise the deal."
He did, however, reveal he would not be signing any strikers on a permanent basis in the January transfer window "unless we lose players".
Meanwhile, Theo Walcott has recovered from illness and is set to return against QPR on Saturday and Wenger believes the Henry news will give the whole squad a boost.
He said: "The players are all happy that Thierry is coming back. You can always learn in life from people who have the knowledge like he has. That was one of the reasons why we brought Jens Lehmann and Sol Campbell back. They have a weight in the dressing room."
Wenger believes Henry will prove to be a loan deal with a difference.
He said: "You do not find players on the planet of this quality who are available for two months.
"I met him when he was 17 years old. I had him here at 22 so I know every movement he can make. He is a super-intelligent guy and he is a special talent.
"In our job, you need to have the body to respond to what your brain wants to do. For a while he had some Achilles problems but he is fit again.
"He knows where to be, knows how to give a problem with the opponent. You never lose that."
Wenger is hoping that Henry can make a similar impact to Campbell when he returned.
"Sol was fantastic when he came back. Sol is like Thierry. Once they are committed you know you can rely on them.
"I remember when we went to Tottenham, Sol was absolutely amazing for a guy who, three weeks before, you wondered if this guy can still play in the Premier League. He had such a capacity to fight. They have that pride. Now we have Thierry."
The news on England midfielder Jack Wilshere's ankle injury is also positive with a February return date a possibility.