He will be a starter, play two downs, but barely be on the field?
It's not really 2 downs. They called it a 2-down player back in the day when Offenses normally only played 3 WRs on 3rd downs.
Damiem Wilson was the SLB in 2018. He played 27.9% of the defensive snaps.
There were 1024 total defensive snaps for the Cowboys in the 2018 regular season.
Jaylon played most of the snaps.
J.Smith 976
Vander Esch and Lee split the WLB snaps (1004 total snaps between them)
L.Vander Esch 784
S.Lee 220
The SLB is out when the Nickel CB is in the game.
CB Brown and SLB Wilson combined for 975 snaps.
A.Brown 689
D.Wilson 286
For 3 positions (WLB, MLB, SLB/N-CB) that's 3 * 1024 = 3072 total snaps.
Jaylon, Vander Esch, Lee, Wilson and CB Brown combined played 2955 snaps.
That just leaves 3072 - 2955 = 117 snaps (average 7.3 per game) for backups for snaps when they played a Dime DB.