Marshall the least effective keeper in the PL

You really aren't doing yourself any favours. Jakupovic conceded one in stoppage time to United at the start of the season, and again in the Cup conceded an (offside) Mata goal and then a second to Fellaini. Marshall conceded more in one match against Liverpool.

Or you go down the other end. Jakupovic conceded a penalty against Bournemouth, while Marshall conceded 6.

 

How many of the goals conceded at Liverpool and Bournemouth were attributable to Marshall?

You may consult your spreadsheets.