Roberts pulled up just after the hour mark in the 47-17 victory at the Sydney Football Stadium and will have a scan on the injury on Sunday, potentially joining England centre Manu Tuilagi and Welsh winger George North in the treatment room.
"The medics thought it might have been a hamstring but he said he has never pulled a hamstring before," Gatland said of Roberts who is a doctor.
"He's not kind of sure, so he's got his fingers crossed that it may not be that. So we just has to see what happens tomorrow."
England's Billy Twelvetrees has been called into the squad as midfield cover and will play against the ACT Brumbies in Tuesday's tour match and Gatland said the news about Tuilagi and North, who can also cover the centres, was hopeful.
"Manu trained yesterday, his shoulder's a bit sore, George's hamstring's still a bit tight," he said. "Just progressing with them with the medical team and hopefully both of those will be in contention next week."
Better news for the coach's midfield options was the performance of Roberts's centre partner Jonathan Davies, who played a part in most of the tourists' best moves and grabbed the final try.
"I thought he was fantastic," Gatland said. "I thought Jonathan, given the quality of our midfield and the pressure, obviously, he was under to respond and that I thought it was one of the best games I have seen him play.
"It's quite nice to come in the change room afterwards and someone like Brian O'Driscoll said 'man, how good was Jonathan Davies?'. It's almost an endorsement, isn't it? Rather than the coaches saying it. I thought he was very, very good."
If Roberts is fit, O'Driscoll is likely to start at outside centre in the first Test against the Wallabies in Brisbane next Saturday.
But if the Welshman fails to recover O'Driscoll would probably move to number 12 with Davies at outside centre where he usually plays for Wales.
"The way we operate there is a lot of interaction and changing and positional play between 12 and 13, so we have played him there on this tour," Gatland said.
"The beauty of what he does give us is a left foot option as well."
There were plaudits too for Welsh lock Alun Wyn-Jones and Irish winger Simon Zebo, who made his debut after coming into the squad as cover for injured compatriot Tommy Bowe.
On a tough night for anybody handing out man of the match awards, Leigh Halfpenny's two tries and additional 20 points from another flawless display of place-kicking was hard to beat.
"Apart from his goal kicking I thought some of his lines of running and his positional play defensively and kicking out of hand, I think he is playing at the top of his game," Gatland said.
"He is the ultimate professional. He prepares and does everything that you would expect of him. I thought again today he was very, very good."