Exiles head coach Lyn Jones spent five seasons in charge of Henson at the Ospreys and is confident he can once again get the best from the Welsh international but he has also urged patience.
"There's a good chance he'll never reach those heights (of his time at the Ospreys) again to be honest with you, but he just needs to get up to speed and establish himself and I'm not really confident he'll become a very, very good player until about maybe mid-October, November," Jones told Sky Sports.
"The guy needs a bit of time, he needs a bit of space, his tools are quite blunt and he'll need to sharpen them up, so he's got a lot of work ahead of him that he's prepared to get on with. He's rolling his sleeves up, and all you can do is offer people opportunities and it's up to them how they take it but we're confident that he can deliver."
The famous club, who contributed seven players to the 1971 Lions tour, are Henson's fifth club in little over 18 months after being released by the Ospreys in 2010 and having short stints at Saracens, Toulon and the Blues.
The Exiles have not disclosed the length of Henson's contract but he will link up with his new team-mates tomorrow.