Rovers have handed Bain a contract until next summer after the Grenada international striker impressed Barnes while on international duty.
Bain played for Tranmere's reserve side at Blackpool on Wednesday night but remains some way off full fitness.
Barnes said: "He is a bit rusty because he hasn't played competitive football for a month, but he looked quick and strong and he did well.
"He has been training in Grenada but it's not the same as playing games. Kithson has come to add to our squad and give us something we lack.
"He hasn't come here to save us on his own with five or six goals a game, it has to be a team effort."