Here's another theory - the undue and petty influences of SAF.
Ferguson picked Nicol for the’86 swuad an played him in midfield in each game.
How he didn’t get picked for ‘90 is more of a mystery.
‘86 squad was still Miller & McLeish at CB, with Maurice Malpas, Roy Aitken, Richard Gough, Arthur Albiston & David Narey making up the defence and the likes of Eamon Bannon & Jim Bett included as midfielders - but as mentioned Nicol played all 3 games.
‘90 - McLeish, Malpas, Aitken, Gough & Bett still in the squad, with Craig Levein, Dave MacPherson, Stuart McKimmie, Murdo McLeod, and even Gary Gillespie in the team.
Was Nicol injured? He was 28 and his last cap was in ‘91? Nicol was a regular for Liverpool until ‘94.
I moved to Scotland in 1990 - so I recognise a lot of those players - some of them were good - Scottish football was producing better players in those days (it feels) - Craig Levein, in particular, was a very underrated CB who got done over by injury and the Celtic fans would speak very warmly about Roy Aitken - but Nicol seems to have been weirdly overlooked.