I wrote about it several times: there is no player comparable to Mane and Salah who will come here to "take the strain" off them. We can try to buy another winger who is OK with being a back-up – but we already have this in Shaqiri and the best we can hope in this scenario would be a marginal improvement. I think Shaq is quality and his attitude has been spot-on so far; the only intractable problem I can see is that he can't play on the LW at all and can't lead the line either – so Salah and Firmino have to be always shuttled around to accommodate him, which is not ideal. So just based on that lack of versatility, a marginal improvement on him is possible, but it's not all that easy to find someone who is both good enough and willing to play this role – and won't break the bank.
One thing you and other posters are underestimating, IMHO, is that we don't have to play all 3 of Salah, Mane and Firmino in every game to win, including big games. If you have a really solid defense and a midfield that won't be outplayed, 1 in-form attacker is generally all you need to get your 1:0. We beat Barca with only Mane available from the front 3, we beat PSG with Firmino coming off the bench and won the CL final with him playing far below his level; Sturridge started must-win games against Huddersfield and Newcastle in the decisive part of the title race. In the last couple of seasons, Klopp has been very careful to give each of the front 3 a game off here and there and it didn't seem like there were overworked – besides all 3 seem to be the type who need to play all the time to maintain their sharpness. Would a long-term injury to one of them make things a lot harder – of course it would. But that's just like saying Barca needs a like-for-like replacement for Messi in case he gets injured. There is no replacement; if something like this happens you just try to cope with it. Like I said many times before, I think what we need the most is to add more goals, creativity and quality in midfield to supplement the front 3 – and that would also come in handy when one of them is unavailable.