Three points I would make on Torres.
1) In regards to his physicality, it's important to remember that he can actually bulk up when moving to the PL, as someone like Vidic had to do when he arrived at a similar age. The important physical difference between Torres and Lindelof currently and permanently is that the former is noticeably taller, which puts him at an advantage.
2) In terms of ability on the ball, he is more than slightly better than Lindelof.
Looking at it statistically one might think their passing completion percentages suggest otherwise, with Torres 89.6% on while Lindelof is slightly ahead on 91%. But a closer look shows a difference is how they pass.
In terms of per 90min:
Progressive Passes: Torres 3.83, Lindelof 2.91
Progressive passing distance: Torres 395.73, Lindelof 356.58.
Passes under pressure: Torres 6.35, Lindelof 5.03
Long passes attempted: Torres 14.62, Lindelof 9.80.
Etc, etc. In other words Torres is much more progressive in terms of his passing game.
In combination, he's also much more aggressive in terms of how he carries the ball.
Dribbles attempted: Torres 0.82, Lindelof 0.17
Dribbles success: Torres 85%, Lindelof 37.5%
Players dribbled past: Torres 0.74, Lindelof 0.08
Progressive carry distance: Torres 211.86, Lindelof 176.76
It becomes clear at that point why Torres is so rated for what he does on the ball. He offers a lot more in that regard than someone like Lindelof.
3) In terms of aerial ability, the basic stats would again suggest an issue. In terms of aerial duels won in their domestic league this season: Torres 60.6% , Lindelof 65.7%.
The issue with basing our opinion off that one stat becomes clear when we take a deeper look at their previous seasons' data though.
Comparing that same stat over previous seasons for three key defenders.
Lindelof: 33.3%, 65.1%, 66.0%, 65.7%.
Kounde: 68.8%, 54.9%, 74.5%, 63.8%.
Torres: 47.4%, 60.6%.
Three points to note there.
1) We are basing our assessment of Torres off a very small sample size.
2) The older, more experienced and settled Lindelof has established a more stable baseline for what we can expect in this regard than the younger CBs.
3) Significant improvements are possible, particularly for the younger CBs. We see it in the difference between Torres this season and last (47.4% vs 60.6%), Lindelof in his first season at United versus his second (33.3% vs 65.1%) and Kounde's rather erratic changes in success rate both up and down.
When you consider the rapid improvement we've seen from him in the space of one season allied with Torres' physical advantage and potential to develop further physically, it wouldn't be at all surprising if he immediately proved to be better aerially than Lindelof despite that one stat from this season. I certainly wouldn't take it as read that that one stat from one season provides an accurate representation of his ability in that regard. Particularly as (purely subjectively) he doesn't look that vulnerable to me.
None of that means Torres should be opted for over someone like the more ready-made Varane imo. But as an alternative if we can't get Varane, I think he looks very promising. If he were to develop into his frame as Vidic did and/or continue improving aerially as he did this season (and as even Lindelof did upon arriving here) then he could be an excellent CB, particularly as we are a side who look to play out from the back. The issues is that as with someone like Vidic, that would likely mean an adjustment period.