The Hampshire loan signing was kept waiting in the pavilion for his first bat in County colours as persistent rain fell throughout the morning and early afternoon in Northampton.
That meant McManus, who has been brought in for the next four weeks to replace 'unavailable' former skipper Adam Rossington, joined Luke Procter in the middle and the pair enjoyed themselves for the 19.2 overs of play that were possible.
McManus made a measured 39 from 65 balls, hitting six fours, while Procter finished unbeaten on 46 from 98 balls as Northants ended the day against a limited students attack on 530 for five.
The third and final day's play is due to start at the County Ground at 11am on Wednesday, and admission is free for all spectators.
Meanwhile, Northants overseas signing Will Young scored his maiden One Day International century for New Zealand as they hammered the Netherlands in Tauranga.
Young hit 103 not out from 114 balls as the Kiwis cruised to 204 for three in just 38.3 overs to easily overhaul the Netherlands' 202 all out.
Young was in great form, hitting eight fours and three sixes against a Dutch attack that included Northants fast bowler Brandon Glover in its ranks.
The day didn't go well for Glover, who ended with figures of nought for 24 from his three overs.
There are two more ODIs in the series, on Saturday and Monday in Hamilton.
Northants begin their LV= County Championship Division One campaign next Thursday (April 7).