Alex Corbisiero and Jamie Roberts are unlikely to participate in Saturday's second Test against Australia in Melbourne after Warren Gatland revealed they are struggling to be fit for the match.

The England loosehead prop started but could not complete the Lions' 23-21 first Test victory over the Wallabies in Brisbane due to a tight calf and Roberts, the Wales centre, missed the contest because of a hamstring injury.

"At this stage it's a little bit doubtful that they'll be involved," Gatland said of the pair who will be assessed tomorrow, when the team for the second Test at Etihad Stadium will be announced.

"Alex is struggling a little bit and Jamie is probably in the same boat. Both of them should be fine for the third Test, but at this stage . . ." he said. "We have a recovery day tomorrow when everyone's off. Then we'll look at seeing how they are on Thursday morning. The medics have been doing a fantastic job for us and they might be declared fit, but it's doubtful."

Gatland was speaking after the Lions routed the Melbourne Rebels 35-0 in their final midweek match at AAMI Park with Conor Murray, Sean Maitland, Sean O'Brien and Ben Youngs crossing. Glen Jackson also awarded a penalty try in what was a highly-polished performance – topped by Maitland's terrific touchdown – that atoned for last week's dismal 14-12 loss to the Brumbies.

Gatland announced a clean bill of health from the match – with the proviso that nothing develops in the next 24 hours – and also confirmed that Paul O'Connell would be remaining with the team in Australia despite being ruled ruled out of the rest of the tour yesterday with a fractured arm. His positive influence on the camp is such that Gatland is more than happy for the Irishman to remain with the squad.

"Paul has been given the option to stay. I don't think he will be on a Lions tour in four years' time. He played such an important role in our win in the first Test," Gatland said. "A lot of players who have been around for a while haven't had a lot of success in the southern hemisphere and were desperate for it. Paul was one of those. It's a big loss for Paul and the squad because he was playing some of the best rugby I've seen him play in a long, long time. He looked in great shape. The combination between him and Alun-Wyn Jones was starting to look very strong."

The loss of O'Connell has opened the door to Richie Gray, one of the standout performers in the win. He knew the match was effectively an audition for a place on the bench on Saturday with England's Geoff Parling withdrawn from yesterday's match to protect him for the Test.

The 23-year-old lock enjoyed an impressive 80 minutes alongside second-row partner Ian Evans of Wales who was also pushing his case for inclusion. Gray, who earlier in the tour had been backed by many as a first Test starter, is optimistic he will get the nod.

"The focus was just to keep improving. This was the team that largely played last Tuesday [in the 14-12 defeat to the Brumbies] and we were pretty upset without ourselves. We wanted to come out and prove a point and keep the momentum going. Obviously, Paul getting injured was very unfortunate because he's huge for us, but it opens up an opportunity for Ian and me. Ian played very well, his line-out was exceptional, so we'll see what happens."

The final decision on which one provides back-up to Parling and Wyn Jones this weekend will come down to tactics, according to Sir Ian McGeechan. The former Scotland coach – the last man to lead the Lions to a Test series victory, against South Africa in 1997 – was impressed by both Gray and Evans but believes they both offer something different.

"The two second rowers worked really hard and I think it will be tactical decision from the coaches as to which way they will want to go. Do they want to go with the dynamic Richie Gray, who gets into the outside spaces a little bit earlier? Or the power of Ian Evans?"

Gatland specifically namechecked Gray afterwards, adding that he was delighted with the level of his players' performance against a committed Rebels side.

"That was a good response to the disappointment of last week. We said beforehand it was an opportunity for players to put their hands up," he said. "Richie Gray had his best game on tour, Richard Hibbard carried the ball well, Conor Murray and Ben Youngs were good, as was Sean Maitland. There's still an opportunity for people to be involved because there are two games.

"Given the attritional rate, there are bound to be a few injuries between now and the final Test. Even if you're not selected or involved in the next two Tests, you can still feel proud of having been part of a successful Lions tour. That's what the aim is."