[nodamage]

I've seen this quoted a lot lately but saying that a literal monopoly (as in, a single company with 100% market share) is not required doesn't really say much about what is required, which depends on the specific violation being alleged.

Some violations (price fixing, bid rigging) are considered so egregious that they are illegal regardless of market share.

Other violations (tying) require some degree of market power.

Violations related to monopolization or attempted monopolization do require monopoly power (or the dangerous probability of obtaining it it).