There's 1 area where Lindelof (and Bailly) are particularly and noticeably weak.
This is average amount of aerials won per 90 mins from players playing CB this season in the premier league.
If we ignore the players who only played 1 game, Bailly and Lindelof are 2 of the 5 CBs with the least attempts of winning the ball in the air in the league (2 and 4 respectively).
Bailly has actually won each of the headers he's challenged for so at least he has that going for him. Lindelof on the other hand has lost a decent amount of the duels he went for as well. At least its not as bad as Toby I guess but the numbers are similar and Toby simply attempted to win more than Lindelof.
For reference Smalling averages 6.5, Jones 5, Luke Shaw 3.6 from left back, Pogba 3.5 from CM, Rashford 2.9 and Sanchez 2.8 aerials per 90 mins.
Against different teams with different tactics this can either be a huge deal against teams with good threats in the air, or against teams that keep the ball on the floor it may not be as big of an issue. But it is certainly a weakness with Bailly and Lindelof and they will need teammates to cover them.
It also doesnt help that De Gea's weakness is commanding his area on crosses or coming for balls inside his 6 yard box. So its doubling down on something that, if you're an opposition player playing against United and you're okay in the air, you'll fancy yourself to get something from challenging in the air against Lindelof and Bailly as well as De Gea in goal.
De Gea probably needs CBs who are strong in the air to cover his weakest area, while Bailly and Lindelof probably need a goalkeeper who is strong coming for crosses and a CB partner who is great in the air.
Personally I still think Smalling & Lindelof is our best option most of the time for the rest of the season. But longterm I think we need another player who does well in the air to help with our weakness.