I understand what you're saying Macca, and I'm not implying that any skill can be improved simply by putting in extra work, but adding muscle isn't in that category (if that is indeed Lallana 'problem').
It's complex. There's far too many things to factor in. You can work on general fitness, half of the issue with strength is stamina, you build up your match fitness and you work to increase your ability to cover ground over long and short distances, to be sharp and first to the ball. Half the battle isn't out-muscling your opponent, it's about being quicker in the the first instance to give you a standing chance. But then this includes game intelligence too, you've got to be sharp in mind to react.
Coutinho has pace to get himself away from players when he turns them inside out, over that first yard or two he has that ability to break away enough to get in full flow and either release the ball, or run at teams and force them onto the back foot. I think Lallana does that fairly well too when he's fully fit and sharp, which i think last night went some way towards. His biggest issue for me isn't so much his strength (collectively it might be a problem, but individually it's not), it's his ability to do all the good work, by getting himself out of tight areas and into good positions on the offensive, only to then lack that instinct to pick the right pass or go for the right option. He
can do it, but he doesn't do it enough.
Coutinho suffers from the same issue, but he has that extra bit of class at times in terms of vision and the ability to pull something out the bag. Both are really good players who will be important for us this season, but it's crucial that they both smooth out their rough edges. Like I say though, I don't think strength is the big issue here.