To understand why Solskjaer opted for Lindelof + Maguire, rather than Smalling + ?, you have to look at De Gea's passing stats.
De Gea this season has a pass completion rate of 71.7% in the PL. Last season it was 60%. The season before that 57.5%. Before that 60%, 56.9%.
A more detailed breakdown into short passes and long balls.
Accurate Long balls 3.1 - 5.3 Inaccurate Long balls
Accurate Short Passes 10.6 - 0.1 Inaccurate Short Passes
Figures for last season were 7.9 - 9.8 (Long) and 7.1 - 0.2 (short)
Figures for the season before 7.0 - 10.8 (long) and 7.6 - 0 (short)
So, in simple terms, De Gea is required to pass the ball less and when he does pass, he favours short passes over long, because his success rate on long passes is down, due to the lack of Lukaku, Fellaini and before that, Zlatan. Maguire and Lindelof are far happier to receive a short pass from De Gea, then take responsibility for starting moves, than Smalling, Jones and Bailly were.
Smalling did not make the 18 man squad for our first three PL games this season. Tuanzebe was the chosen substitute central defender. That should tell you a lot about how Solskjaer wants the team to play.
As for defensive errors, Lindelof has been the main player at fault for 2 PL goals conceded this season (Palace first goal and Southampton's goal). Maguire's failure to provide cover or play offside also contributed to Palace's first goal, and his unwillingness to rush out to block Matty Longstaff's shot, left De Gea unsighted and not enough time to react when the ball went past Maguire. Maguire also made two careless errors that eventually led to the corner from which Wolves scored.
Smalling started 13 PL games under Mourinho last season and 11 under Solskjaer. During Solskjaer's time in charge, Smalling was at fault for 2 of Everton's goals, one as the main player at fault, the other as a contributor. Under Mourinho, Smalling conceded a penalty against Everton, he was poor at the Arsenal corner that resulted in a huge De Gea blunder, he was beaten in the air by David Luiz, who hit the post, in the build-up to Chelsea's late equalizer, also a weak challenge from Smalling failed to prevent one of Spurs' goals at Old Trafford.
If the manager wants to play a certain way, he needs players that suit that approach. Lindelof lacks the physical strength and ability in the air that most CBs possess but he is normally comfortable receiving possession. Maguire is good in the air, wins most physical duels, is comfortable in possession but painfully slow, which leaves us horribly vulnerable to counter-attacks.