I think his loan at Wigan helped Koeman decide he was worth a chance ahead of Galloway/Garbutt as left back cover for Baines. He started off with a bang for Wigan, two goals in his first match , MoM awards , looked good as an attacking full back when they played United in the cup , then Wigan got involved in a relegation dog fight and he had to knuckle down defensively. Since then in his games for England U20s and his three appearances in pre season for us he looks like he's now better defensively but less inclined to go forward in attack, though in his last game for us he did look like his attacking appetite has returned. As a result Kyle Walker-Peters got the no 3 position for England and in fairness looked a much better attacking full back than Connolly. He's not an out an out dribbler but shows good positional sense when going forward and showed in his last game that while he's naturally right footed he can make a decent cross with his left foot. It's a difficult one for the lad in that he's not yet good enough to replace Baines, would provide good cover if he did need to come on, but RK is likely to go for a bench that includes Jags and one of either Holgate/Kenny/Martina to cover whoever plays RB . So first team minutes may be difficult for him.
I wouldn't send him out on loan though. While it worked at Wigan, I think a spell training specifically with the first team under RK , ironing out weaknesses , learning PL patterns etc would benefit him. I know the U23s train with the 1st team, but I imagine that it's a step up in respect of practice , moves , positioning etc when potentially you could be playing on Saturday if an injury occurs. Hope he makes the bench regularly.