Jamie Roberts: Happy 100 to Gats! I owe our coach at Wales a lot and it's a pleasure to work with someone who says it how he sees it

The game against Tonga is Warren Gatland's 100th as an international coach.

I owe the guy a lot – he gave me my first Wales cap back in 2008 and I've been lucky enough to play consistently in the side for the five years since then.

If you're dissecting him as a coach, I think the best attribute you can give him is his complete honesty. If you're playing badly, he'll tell you – it's as simple as that. But also he'll tell you when you're doing well. With him, you know your standing.

Yes, he likes mind games and he knows just how to push certain people's buttons, and that's one of his great strengths. Ruthless honesty is exactly what you want as a player, for me it's the strongest attribute that any coach can have.

That honesty means you can be on the receiving end of the hairdryer treatment and why not? It works to good effect. Between him, [and coaches] Sean Edwards and Rob Howley, I think every player in the squad has had the hairdryer at one point or another, and that's the way it should be.

While it's a century for Gats, it's game one for Hallam Amos on the wing. A week ago, I said I saw a lot of myself in another debutant in Cory Allen, now sadly out injured; this week it's the same with Hallam, but that's more to do with our medical backgrounds.

When I made my debut for Wales I was a third-year medical student, Hallam's only in the first year of his medical degree, so what he's done is pretty awesome when you think this summer he was only just sitting down for his A levels, from which he came away with straight As.

I remember sitting down with him a month before his exams at the team hotel – he got in contact and said he wanted a chat – and, by then, I knew he'd applied to study medicine. He really wanted to know how those years of mixing university and rugby were, and what to expect really.

Most people were telling him that he couldn't do the two to the best of his ability but I said otherwise, although in terms of advice I said it was about doing it his own way and making it work. For me, playing rugby helped me through my medical degree, and the medicine helped me in the rugby.

From the playing side of things, this is simply a latest opportunity he needs to grab with both hands. He needs to come off the pitch having given it everything and he'll be helped by the experienced players around him, in particular the rest of the back three in Leigh Halfpenny and George North.

For Hallam and some of the other players brought in for the Tonga game – there's been 11 changes in all – it's a chance to push for selection for that last game against Australia.

I've said it in previous columns but blooding these youngsters is a great thing with the World Cup in mind in two years' time. No disrespect to Tonga but this is an opportunity for the coaches to develop strength in depth in every position.

You look at, say, lock and you see that's not a problem. Alun Wyn Jones and Bradley Davies have done their bit and in come Ian Evans and Luke Charteris, both guys with immense experience, plus there's Andrew Coombs who can play in that position. But that's not the case in every single position.

I remember at this stage two years out from the last World Cup, players like Sam Warburton, Jonathan Davies and Toby Faletau were only just winning their first caps and you only need to look at how integral they've become to see the value in blooding youngsters at this stage with one eye always on the World Cup.

The captain for the Tonga game, Ryan Jones, has talked this week about integrating young players in the team. He's always been great at that. As a young player, he was a guy I looked up to and tried to learn as much as I possibly could from.

Wales always have the ethos of players taking responsibility, of 15 leaders on the field but, as captain, there's no denying that Ryan, much like Sam Warburton in the role, leads by example. They're both the sort of players who put their bodies on the line in a big way and that's inspiring. Ryan's very experienced in that leadership role, and he's very much a people person. He can get the best out of the young guys in the side.

Among the other experienced guys in the side is James Hook at fly-half. He's not available for the Wallabies game – he'll be returning to the Top 14 with Perpignan – but he will be keen to put question marks in Gats' head with the Six Nations in mind.

This is a guy who's done it at the highest level for Wales and the Lions, and he's a quality player and a quality guy as well. He's very talented, so unbelievably skilful and he's great fun to have around too.

I've known him a while now – we played Under-21s and Sevens together, and I know what a threat he can be with the ball in hand. He's had 70-odd caps and played fly-half, centre and full-back. But the problem I suppose is that he hasn't become first choice with any of them for Wales over the past few years. So for him he'll see this as another great opportunity.

Wales are in a good place right now. I thought the boys played well against Argentina but, fantastic as Wales' play was, I do worry about the Pumas. I only hope they're just shattered at the end of a long season but they didn't bring enough for me, whereas Wales were unrelenting to the final whistle. You could see how much the guys were enjoying playing and just wanting to keep on running in tries.

Tonga are a slightly different prospect. The real danger with sides like Tonga is that you chuck the ball around too much, that you play with too much of a Sevens mentality. Obviously, that can work for you if you're accurate, and you can then cut teams like that apart.

But if that doesn't work, Tonga absolutely pounce on that and they're just so ruthless in terms of their physicality and defensively. So if Wales put width on the ball too early on, when they don't really deserve to, that's when and where they can come unstuck. So I think the message from the coaches is to maybe take your time, have a kicking strategy and then take it from there.

And if I had to make a prediction – and don't pay too much attention to me as I was woeful on that front last week – I'd go Wales 37 Tonga 6.