Defender McLean twisted his knee in the 1-1 draw with St Johnstone earlier in the week and midfielder Armstrong is banned after his red card at McDiarmid Park.
Midfielder John Rankin returns from his suspension, while winger Gary Mackay-Steven is doubtful after missing Monday's draw with a hamstring injury.
Aberdeen manager Craig Brown takes charge of the team for the final time as he prepares to move into the Pittodrie boardroom.
His hopes of bow out on a high are boosted with the return of defender Gary Naysmith and forward Jonny Hayes.
Dons defender Andrew Considine is absent along with midfielders Isaac Osbourne, Stephen Hughes and Chris Clark.