Defending champions India are on top of Group B after Sunday's 124-run thrashing they gave to Pakistan, and a win against the islanders on Thursday will seal their place in the semi-finals of the eight-team tournament.
But Harbhajan, in his column for the International Cricket Council (ICC) website, cautioned India against complacency and expected the men-in-blue to pull up their socks in the fielding department. Sunday's encounter witnessed a few dropped catches.
"India will carry a lot of positives into Thursday's match. Almost every batsman fired in right earnest during Sunday's showdown against Pakistan. Even the bowlers had a fruitful outing, but I would like to see some improvements in the bowling department," the offie wrote.
"I must admit that I was a little surprised by India's underwhelming display in the field. Some of those catches should not have been dropped, but I guess the support staff will put the boys through their paces, and I hope to see an improved performance against Sri Lanka."
"The men-in-blue should also be wary of the complacency factor. On a high after the 124-run win over Pakistan, Virat Kohli and his side must not let the intensity levels down. India just needs to get the basics right. I expect the top order as well as Kohli and Yuvraj Singh to take off from where they left in Birmingham," he added.
The Punjab spinner also backed the Indian bowlers to utilise the conditions at The Oval well.
"I also hope to see Jasprit Bumrah, Bhuvneshwar Kumar and Ravindra Jadeja make the most of the conditions at The Oval. Given the form that India is in, it is likely to emerge winner even if it plays to 70 per-cent potential. But let's just hope the players give it their all," he said.
Commenting on Sri Lanka's current form and the way they lost to South Africa in their opening tie, Harbhajan said the islanders will desperately miss the services of veteran Upul Tharanga, who was handed a two-match suspension for over-rate offences. Leaving behind spate of injuries, pacer Varun Aaron seeks fresh beginning
"To lose a player to suspension is a setback in itself. When the said player happens to be your captain, the team's misery only gets compounded. Upul Tharanga's two-game suspension for over-rate offences is bound to hurt the team as it prepares to take on a pumped-up India," he said.
"The islanders do not boast a great team, and Tharanga's absence will hurt them immensely. With first-choice captain Angelo Mathews injured, it was left to Tharanga to stand in for him, but now he has left a void."
"Let's not forget the upcoming game is extremely crucial from Sri Lanka's point of view because the team has already lost to South Africa and faces a must-win situation against the defending champions," he added.
Harbhajan also had an advice for the Indian batsmen when they face veteran Lasith Malinga, who can be lethal against any side on his day.
"Sri Lanka has the spunk of Niroshan Dickwella and the ever-reliable Lasith Malinga is capable of making any batting line-up look pedestrian on his day. I hope India plays him out safely."
On the English weather, which played spoilsport in two matches so far, besides affecting the India-Pakistan match, Harbhajan said India holds the upper hand if there are no weather interruptions.
"Lest we forget, the English weather is often unpredictable. We have already lost two matches to rain, and I sincerely hope the clouds stay away and we get a full game. If it ends up being a 50-overs-a-side game, then I don't see Sri Lanka troubling India at all." CT 2017: India look to book semis berth against Sri Lanka
"However, if the match is reduced to a 20-20 or a 30-overs-a-side game due to the vagaries of the weather, then the match could go either way. Needless to say, dealing with the Duckworth-Lewis-Stern method is not easy," he added.