Derek McInnes is confident Lewis Ferguson will bring edge and aggression back to the Aberdeen team.
Ferguson is set to resume training with the squad today having missed the last three games.
The influential midfielder will return to action in Saturday’s Premiership clash at St Mirren.
Ferguson and McLennan both tested negative and rejoin their team-mates at Cormack Park today.
McCrorie, who tested positive but was asymptomatic, won’t be back until the home game with Ross County on December 12.
On his star midfielder’s return to the Aberdeen fold, McInnes said: “Ferguson will give us a lot of what was lacking in that first half at St Mirren.
“Having that type of aggression and competitiveness is important for every team.
“We lacked aggression in the middle of the park at St Mirren and aggression throughout.
“There is no doubt we have felt the effect of key players not being available to us for the last three games.
“That natural aggression, competitiveness and desire to go and get your hands dirty wasn’t in evidence against St Mirren right across the board in the first half.
“I can understand mistakes can be made like the winning goal.
“I can understand players maybe cannot be on point every week and play to form.
“However, we still expect to be far more difficult to play against and we were far from that in the first half.”
Aberdeen were furious that three of their players were ruled out due to the Scotland U21 squad Covid-19 outbreak –missing three away games in six days.
The loss of the Aberdeen midfield partnership of Ferguson and McCrorie was particularly costly to McInnes as the bite both bring was lacking in their absence.
McInnes called for reassurances the SFA coronavirus protocols would be more robust at future international meets.
SFA Ian Maxwell subsequently apologised to the teams affected by the loss of their players.
Eleven Scotland U21 squad members were instructed to self-isolate after three – McCrorie, his brother Robbie, on loan at Livingston from Rangers, and Celtic’s David Turnbull – tested positive for coronavirus.
The positive tests followed the players’ returns to their clubs following Scotland U21s’ defeat in Greece in a Euro U21 qualifier.
A Scotland U21 back-room member had tested positive on the Sunday before the clash with Greece and did not travel out to Athens.
McInnes is delighted to have Ferguson and McLennan back.
He said: “They have been doing individual work in their houses with treadmills, but it has all been in isolation for them.
“We will have McCrorie back for the following week.”
McInnes also has Scott Wright, Dylan McGeouch and Marley Watkins out injured.
Although Jonny Hayes and Niall McGinn started against St Mirren at the weekend, it was their first start for a month due to injury.
Right-back Shay Logan was suspended for the Betfred Cup tie, but returns at the weekend.
McInnes expects his team to be stronger on Saturday.
He said: “We have to make sure that with those extra bodies we have a far more competitive team on the pitch.”