Thompson, who was man of the mach in St Helens’ Grand Final triumph, went off against a Tongan Invitational XIII after only 12 minutes with a rib injury while Gildart dislocated a shoulder on 59 minutes.
“I don’t think they’ll play again in this series,” Bennett said at the post-match press conference. “One’s got a rib issue and other a dislocated shoulder.”
The absence of Thompson for next Saturday’s first Test against New Zealand could open the door to Ireland prop Joe Philbin or his Warrington team-mate Jack Hughes while the loss of Gildart, the squad’s only specialist centre, will provide an opening for utility back Jake Connor.
“I’m happy with the squad right now,” Bennett said. “We can cover, it’s not panic time for us.”
Great Britain, making their first appearance since 2007, never got a foothold in the game against a fired-up Tongan team determined to build on their remarkable 2017 World Cup exploits.
They trailed 12-0 at half-time and were 14-0 down when John Bateman scored their only try.
“We didn’t fall down in effort,” Bennett said. “We fell down a little bit with execution, being patient, our kicking game, they are the areas that really hurt us in the end.
“Defensively I thought we were really good. We’ve just got to pick up a little bit with our patience and discipline with the football.
“They are a quality football team, there’s a lot of quality players in that team, I don’t think any of us in the Australian comp doubted for one minute that we were going to have a tough game on our hands and it proved to be that.
“They have been building for the last three or four years. they were formidable tonight.
“They had a little luck as well out there, we didn’t have a lot of luck tonight.
“They were no better than we were until they got that first try and there was a bit of luck involved in that.
“We were up against it at times. they were getting good momentum and we weren’t kicking great.
“In the second half we turned the ball over in vital field positions and we were under a lot of pressure.
“We gave away three seven sets of tackles and it takes away all your energy to do anything with the football, you spend so much time defending.”
Bennett singled out Bateman for praise but was unable to pick out any other players that impressed him.
“I thought John Bateman was coming into his own in the second half a fair bit,” he said. “Outside of that, I wasn’t getting excited by anybody.”
It was a disappointing way for Great Britain captain James Graham to mark his 50th international appearance.
“At half-time we firmly believed that we were still in the fight,” Graham said. “We knew we had a massive job on our hands but we spoke about how we could get back into the game.
“I thought the Tongans played with the lead very well in the second half.”