Do yourself/somebody an injustice
Definition 1 of 1
Pro Tip 1/3Self-belief Impact
Using the phrase indicates a lack of confidence in oneself or someone else, often wrongly. 
By saying you can't lead the project, you're doing yourself an injustice.
Pro Tip 2/3Positive Encouragement
It's used to reassure someone that they're more capable or deserving than they believe. 
Don't do yourself an injustice, your ideas are always innovative.
Pro Tip 3/3Humble Perspective
People often use it modestly to suggest they or others may have been underestimated. 
He did himself an injustice when he said he was an average writer; his work is excellent.