Let's say the worst case happens and Petro walks; how would you like the Blues to proceed? They'd still be tight against the cap with Dunn to sign (though being an RFA with no arb. rights he will likely be only due a modest raise). It would be interesting to see what DA would do in that situation. Do they try and shore up the LHD? Are Bozak and Allen still shipped out (seems less likely). It will be interesting, some chips are going to have to fall this summ-- autumn? Whenever UFA is.
I would explore the UFA market and make a few competitive offers. By "competitive offer" I mean an offer that is in the ballpark of market value, but noticeably less money than the highest bidder. If we can find a UFA we like who is willing to take a discount to step into a top 4 role on a contender, then sign him to shore up the D and move Allen and/or Bozak to make the money work. If it requires market value, then move on and use internal options to construct the blue line. Losing Petro hurts a ton and drops us from an absolutely elite blue line to simply a good blue line. A UFA D man likely doesn't put us back into elite territory, so we absolutely shouldn't overpay for one (especially when we weren't willing to overpay Petro). UFAs I would target to try and get at a discount below UFA value: Krug, Brodie, Gustafsson, Dillon. Realistically, I think it is unlikely that we land any of those guys for a contract that makes sense.
Assuming none of them work out, I wouldn't move an asset for a D via trade unless the value is just too good to pass up. Barring a "no brainer" trade opportunity, I would construct the blue line internally and this would be my tentative plan:
Vince Dunn: if Petro is gone, Dunn has more value to this organization than as simply a good D man on a cheap 1 year deal. Without Petro in the mix, I'd be trying to get a multi-year deal done. We should have plenty of cap space to explore this avenue because...
I would still seriously explore moving Allen and Bozak. With that said, if we are just talking about a Dunn extension instead of Petro and Dunn extensions, the desperate need to move cap space goes way down. It also allows us to retain salary on one or both of them. Even with a flat cap, both of them have serious trade value if we are holding $2M+ of their cap hit and real dollars next season. This is my rough outline of the tentative opening night forward roster next year:
Schwartz-Schenn-Tarasenko
????-ROR-Perron
????-Thomas-????
????-????-Steen
The way I see it, we have 5 undefined roles in the lineup that I have filled in with question marks. Without Bozak, those 5 roles would get filled with some combo of Sunny, Sanford, Blais, Kyrou, Barby, MacMac, Kostin or "other." That's 7 guys for 5 spots. You have 4 guys in that group who have demonstrated an ability to fill at least a 3rd line role and 2 of them have played in that 2LW spot with success. If Thomas graduates to the top 6, then you have something like this:
Schenn-ROR-Perron
Schwartz-Thomas-Tarasenko
????-Sunny-????
????-Barby-Steen
Blais, Kyrou, Sanford, Mac and Kostin are the 5 guys competing for 3 open roles.
Ignoring the cap, now is the time to move Bozak just from a roster management standpoint. If you can retain $1.5-$2.5M of his salary, you should be able to get a nice asset back for Bozak that either largely or completely offsets the fact that we don't have our 2020 or 2021 2nd rounders. If you think Husso is ready, you can explore similar trade options with Allen. Allen with $1.5-2M retained should have genuine positive trade value. Shop him and then decide if that value back is worth the risk of naming Husso the backup. In a world where we let Petro walk, I view us as a good playoff team but not in the top group of true contenders. I's rather see Husso as an NHL backup that season than pushing it a year when we could be true contenders if Kyrou/Thomas/Dunn all take the next step.
Edit: If Petro walks, I'm not pushing hard for a Cup next year. I'm still icing a good team that should comfortably make the playoffs and that would be expected to win a round or two. But I would not be pushing to get them over the top into "true contender" status because I don't see a way to do that without seriously mortgaging the future. I would be focused on growing guys like Dunn, Thomas, Kyrou, Kostin and Mikkola in a winning environment while setting up the team to truly reopen the window in 2021/22. I think moving Allen and Bozak for futures assets to help bolster a depleted prospect pipeline is more beneficial to the 5 year plan of this organization than their contributions to a 2020/21 team. I don't think Boazk/Allen put us into contender status, so I'd be looking to leverage the cap flexibility we suddenly have and the huge revenue stream from the Cup to get good returns for them from teams who are struggling to make cap and/or payroll work.