One may be better off getting a taxi. It should not cost more that a few Euros.
However, if you are feeling confident, there is a bus route Nº. 5 that runs a circular route that passes by the Stadium.
Confusing Timetable here: http://bit.ly/1iQx4W9
To confuse matters further, it refers to Coimbra B as "Estação Velha"
It seems to run +- twice every hour on Saturdays(Sabado) and once every hour on Sundays(Domingo)
Times shown appear to show departure times from Pedrulha bus stop.. Estação Velha is two stops after that.
The bus stop at Coimbra-B/Estação Velha is down the little road that leads from the station(on the town side) and over the roundabout keeping to the left hand side.