My aim in this paper is to provide evidence from diachronic linguistics for the view that some predictable units are entrenched in grammar and consequently in human cognition, in a way that makes them functionally and structurally equal to nonpredictable grammatical units, suggesting that these predictable units should be considered grammatical constructions on a par with the nonpredictable constructions. Frequency has usually been seen as the only possible argument speaking in favor of viewing some formally and semantically fully predictable units as grammatical constructions. However, this paper presents evidence from historical linguistics, and more specifically diachronic arguments of a syntactic nature, to support the argument of frequency. I demonstrate that contrary to other Medieval Greek future periphrases, the Early Medieval Greek form [méllo (‘I am about to’) INFINITIVE (INF)] was both semantically and formally predictable. Despite this difference, [méllo INF], like the other future periphrases, seems to be highly entrenched in the cognition (and grammar) of Early Medieval Greek language users, and consequently a grammatical construction. The syntactic evidence speaking in favor of [méllo INF] as a grammatical construction consists in: 1) [méllo INF] is more resistant to restructuring than many other verbal periphrases with an INF; and 2) in LBG [méllo INF] seems to have grammaticalized — as one of its uses — a specialized function embedded in subjunctives, in which méllo was semantically empty. Both these developments indicate that despite being both formally and semantically predictable, [méllo INF] was highly entrenched in EBG grammar.