There is no question but that I.A Richards influenced McLuhan in multiple respects, lifelong. But the great lesson to be drawn from their relationship concerns their fundamental differences.

These regard:

1. Time: for Richards, time is singular and diachronic; for McLuhan, time is plural and both diachronic and synchronic ‘at the same time’.

2. Ambiguity, synaesthesis, complementarity: for Richards these are results, something produced; for McLuhan these are original — the pro-ducing.

3. Light: for Richards light is thrown on things by human activity; for McLuhan humans are illuminated by light which comes through things to them. 

4. Finitude: for Richards, finitude precludes grasp of (gen obj) the whole; for McLuhan, finitude is the sign of the grasp of (gen subj) the whole.

5. Language: for Richards, language is a human invention and tool; for McLuhan, language is the environment within which human existence unfolds.

Examples can be found in both Richards and McLuhan which would seem to fall on the side of the other (as defined above).  Whenever this is found to occur, the prediction here is that the example will prove to be secondary — something subject to deeper explanation in terms of the above differences.