There had been speculation over the summer that Ibrahimovic would return to Old Trafford and manager Jose Mourinho hinted as much during a press conference.
"Fantastic guys, great club".
Although Ibra won the Europa League and League Cup last season, Henry reckons his motivation to "conquer England" has brought him back to Old Trafford.
The 36-year-old bagged 17 goals in the Premier League last season before his untimely injury ruled him out of any football action for a number of months, subsequently forcing him out of the remainder of the season.
A video posted on the Manchester United official Twitter account showed Ibrahimovic working in the club's gym following his knee operation.
Ibrahimovic, who will wear the No. 10 shirt this season, said: "I am back to finish what I started".
"Challenges, they are nothing new".
"I don't know", he said. "I decide when I'm done".
"Giving up has never been an option".
The Swede will rejoin a squad which has already been heavily boosted with reinforcements in the shape of Nemanja Matic and Romelu Lukaku. I was happy. It's not always about the sportive thing. "I think he wants to do more, and he still didn't win the league", Henry told Sky Sports.
"Signing Lukaku makes us even stronger because he has different qualities from me as a striker and from (Marcus) Rashford as a striker".
After the contract extension was confirmed, Mourinho said, "We are delighted Zlatan is on the road to recovery and we are equally delighted to have his ambition and experience back with us". "I have no doubt that he will be important in the second part of the season".