Without Cooper this season, Plymouth conceded an average of 1.7 goals/game and kept a clean sheet in 19% of games. With Cooper, the average goals conceded was 1.21 and they kept a clean sheet in 37% of games.
Here's how his stats compare to Viktor Johansson (and Wes/Grbic):
Saves per game: Cooper 3.3 (7th highest in the league) / Viktor 4 (highest in the league). In the Premier League, Wes had 4.4 saves per game (3rd highest in the league) and Grbic 4 (6th highest in the league). I think it's clear that this stat is not the best indicator of ability, but rather the weakness of the teams overall (us, Plymouth and Rotherham were some of the worst teams in the respective divisions) as they simply faced more shots compared to others.
Expected goals against minus actual goals against: Cooper 3.5 (5th best in the league) / Viktor 2.1 (9th). Wes was -2.9 and Grbic was -8.3 for comparison. A positive score suggests better luck/ability, while a minus score suggests bad luck or a lack of ability.
% of crosses stopped: Cooper 11% (2nd) / Viktor 7.8% (4th). Both Wes and Grbic stopped 5.7% placing them about 17th.
Statistically, you'd say Cooper was better than Johansson, but obviously Cooper was out for a long time with a bad injury [insert joke about how he'd be a good fit for us then]. As others have mentioned, Cooper is entering into the final year of his contract (i.e. the same situation Berge/Ndiaye were in at the start of this season), so there may be the possibility of negotiating a decent price.