The sentiment that Lyubushkin should be the one sitting is IMO misguided. The Rielly-Lyubushkin pairing so far has been the best pairing Rielly has been on by far in terms of defense from the high danger areas.
The closest the Leafs had come previously to the Rielly pairing not being a complete mess in the high danger areas was the infamous Rielly-Hainsey pairing, which Babs basically never split up once they were put together (13 5v5 mins per game over 2 seasons, 0.23 HDGA/60 Rel, -0.7 HDGF% Rel, with a 45% Offensive zone start %).
Since then, every pair Rielly has been on has either had to be heavily sheltered or when they have seen usage similar to Rielly-Hainsey, have been caved in:
Rielly-Ceci: 10 5v5 mins per game, 0.24 HDGA/60 Rel, -13.1 HDGF% Rel, with a 51.7% Offensive zone start %
Rielly-Barrie: 6 5v5 mins per game, -0.54 HDGA/60 Rel, +16.9 HDGF% Rel, with a 62.4% Offensive zone start % (heavily sheltered)
Rielly-Brodie: 11 5v5 mins per game, 0.19 HDGA/60 Rel, -9.48 HDGF% Rel, with a 54% Offensive zone start % (North Division numbers not included)
Since Muzzin got hurt on Feb 22nd the Leafs split up Rielly and Brodie. Initially they tried out Rielly-Liljegren, which was an absolutely horrendous pair at defending in the high danger areas (5 mins per game at 5v5, 2.64 HDGA/60 Rel, +1.46 HDGF% Rel, with a 57% Offensive zone start %)
Then Keefe finally tried out Rielly-Lyubushkin, and since then they have been great defensively in the high danger areas (7 mins per game at 5v5, -0.43 HDGA/60 Rel, -7.7 HDGF%, with a only a 37% Offensive zone start %). This is basically a first for any Rielly pairing other than Rielly-Barrie in extremely sheltered usage, meanwhile the Rielly-Lyubushkin pair gets opposite usage in terms of zone starts.