After Lukaku's record £28 million transfer to Everton on Wednesday, Chelsea have been left with on three first team striker for the forthcoming Premier League season.
Stuart believes Chelsea's backup strikers could cause Mourinho some headaches this season with Torres and Drogba both well past their best. Should Costa pick up an injury, the former Chelsea midfielder feels they could struggle in both the Premier League and the Champions League.
He told talkSPORT: “Diego Costa is a magnificent signing, there’s no doubt about his qualities and he seems to have settled in nicely. I don’t see him being any problems.
“The issue Chelsea have, is that, [following Lukaku's departure], unless they can get anyone else in, they will be in the same situation they were in last year. Jose Mourinho doesn’t know who to turn to if anything happens to Costa.
“If he picks up an injury or suspension, where does he go? Will he look to Chelsea legend Didier Drogba? Let’s be honest about it. At his age, if he’s not playing at regular intervals, it’s not easy to step into a Premier League or Champions League game [and perform].”
Despite his legs not being what they once were, Drogba aged 36, certainly still knows where the goal is having scored 14-goals for Galatasaray last season, which is more than he scored in his final two seasons at Chelsea.
Fernando Torres on the other hand definitely cannot really be relied on should Costa pick up an injury. The 30-year-old has scored 45-goals in 175 appearance in all competitions since his £50 million transfer from Liverpool; that means Chelsea have paid just over £1 million per goal for the forward.