Stowers premature departure occurred shortly after his colleague Elvis Seveali'i was sin-binned for a dangerous tackle on home fly-half Sam Vesty.
Toulon, who visit Thomond Park Stadium a week after, recorded their first home win of the season with a 41-10 win over Agen, Jonny Wilkinson scoring 21 of those points including two tries. Wilkinson scored all his side's points in their two previous matches.
Ospreys were at home to Benetton Treviso and they ended up five tries to one 32-16 winners at a rain soaked Liberty Stadium. The Italians were in contention up to the break when they trailed 15-9 but once the Magners Champions upped the pace in the second half there was only ever going to be one outcome.