It means Ayden Duffy, who replaced Marriott after 15 minutes of the match at Chester, will make his first league start on Saturday at Sincil Bank.
Onuora told BBC Radio Lincolnshire: "It's an opportunity for him to show what he can do.
"The only thing you can't buy is experience and that comes with decision making. You're either found out or you flourish."
With the Imps assured of a mid-table finish Duffy, 21, had been told that he would be given his debut before the end of the season, but he admitted that he had not expected it to come in the way that it did.
"As soon as I saw Alan flagging I knew then that I was getting my chance," he said.
"I was slightly nervous to start with but once I got the first shot out of the way I was all right from there."
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