The former Chelsea manager has put the blame on some of the striker's team-mates.
Former Chelsea boss Gianluca Vialli believes Diego Costa is still one of Europe's top-10 strikers and feels the 27-year-old is being let down by his side's service, according to the London Evening Standard.
Indeed, Vialli insists the Spaniard's travails aren't solely down to him, suggesting the quality of service from Chelsea's attacking midfielders has done Costa no favours so far this season.
Vialli is quoted as saying by the Standard: "I think he's one of the top 10 strikers in Europe. He proved it last year and seasons before in spain. He's obviously going through a tough time but it's not entirely down to him.
"Sometimes it depends on the quality of service from offensive midfielders. If I was Diego Costa I would train hard but play easy, which means you try hard without trying too hard."
It's an interesting point from Vialli - but it doesn't necessarily mean Chelsea's 'offensive midfielders' are doing anything wrong.
The Blues are certainly nowhere near as potent going forward this term yet perhaps the likes of Eden Hazard, Willian and more aren't gelling with Costa and don't trust his form enough to pass to him.
During bad games, whenever Chelsea's forwards enter the final third, they look out of ideas. And perhaps that comes from the man spearheading their attack not being in the right positions and not enjoying enough cohesion with his team-mates.
Against Porto, Jose Mourinho's men did the business in front of goal. But the conundrum of Costa certainly remains.