Drawn from Jansenius' Augustinus: man could not be created in pure nature without grace; the grace of love was still grace though fitting
These points are taken from Jansenius himself, who in his posthumous book entitled Augustinus gathers this same teaching from Augustine at length, proves it and enlarges on it. He does so especially in the first and second books on the state of pure nature.
There he argues at length that man could not have been created in a state of pure nature without any grace at all. By the powers of nature alone, even uncorrupted nature, without the help of grace assisting nature, man cannot love God with any true love or be made blessed by any kind of contemplation and enjoyment of God. So if God were supposed to create man in a state of pure nature without any grace, he would necessarily create him both bad and wretched, which is wholly at odds with God's goodness and wisdom.
He teaches besides that the grace of love for God in which the first man was created keeps the full and true character of grace. God gives it to the innocent creature not as a debt of justice but because it befits the creature's weakness, and by the law of his own wisdom, kindness, holiness, mercy and rectitude. Otherwise all sins, and the creature's love spilling down into lower things, would be laid at God's own door, since he commanded it to bind its love to no one but God, its source and end.
These are Jansenius' own words, in book 1 On the State of Nature, chapter 20, whose title is "How the good will in which a rational creature must be made was grace."
Latin
XV. Ista petita sunt ex Jansenio ipso qui in libro posthumo, qui Augustinus inscribitur, prolixe doctrinam eandem ex Augustino colligit, probat, & amplificat. Praecipue vero primo & secundo libro de statu naturae purae, ubi multis argumentis confirmat & ostendit hominem non potuisse in statu naturae purae, sine omni gratia creari: quia per solas naturae, etiam non corruptae vires, absque gratiae naturam adjuvantis auxilio, nequit homo Deum ullo vero amore diligere, & ulla qualicunque Dei contemplatione & fruitione beari. Adeoque si Deus supponeretur hominem in statu naturae purae, sine omni gratia creare, necessario illum crearet & malum & miserum: quod Dei bonitati & sapientiae omnino repugnat. Ac praeterea docet gratiam dilectionis Dei, in qua primus homo creatus est, verissimam retinere rationem gratiae, quamvis Deus eam nullo justitiae debito, sed ex condecentia infirmitatis creaturae, ex lege suae sapientiae, suaeque benignitatis, & sanctitatis, & misericordiae propriaeque rectitudinis, creaturae innocenti largiatur, ne alioquin omnia peccata, & dilectio ejus in inferiora diffluens, in ipsum revolvantur Deum, qui ei dilectionem ex nemine nisi ex Deo principio & fine suo religare praecepit. Quae sunt ipsa Jansenii verba, lib. 1. de statu naturae, cap. 20. cujus titulus est, Quomodo Bona voluntas, in qua condi debet creatura rationalis, esset gratia.