The 20-year-old is expected to come straight into the side for Saturday's clash at home to Welling United.
His arrival has seen Craig Hill leave City, with his final appearance coming in the 7-1 defeat at Dartford last weekend.
United always planned to loan out Stevens, who has not made a senior appearance for his parent club, but spent the first half of last season with City at Marsh Fam.
He finished the last campaign as back-up to Simon Eastwood, but the U's brought in the experienced Scott Shearer this summer to free up Stevens to clock up more playing time.