Since Shreyas' fitness is in doubt, I think it will be risky to have him on the squad. Sanju Samson would thus have been a better choice than a guy with more experience for a major event like the ODI World Cup.
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Shreyas Iyer's replacement for Sanju Samson is also a good option because of some injury during the IPL Iyer was seen in a troubled situation and agreed to replacement for the upcoming ODI World Cup.