Defoe replaced Emile Heskey at half-time in Saturday's 2-1 friendly win over Slovenia, scoring to put England 2-0 ahead with his fifth goal in three England games.
It was his third international goal this season after two against Netherlands in last month's 2-2 draw in Amsterdam while his club form has been equally impressive with four for Tottenham Hotspur.
Capello must now decide whether to start with his preferred partnership of Wayne Rooney and Heskey - or with Defoe and Rooney up front for the match that will see England qualify for next year's World Cup finals in South Africa if they win.
"When Defoe plays in the second half he scores," said Capello. "I have to choose the side. I have time to decide."
Defoe said: "Everyone wants to play; no one wants to sit on the bench. But if I can come on and make an impact like I did, I'll do that. At the minute, everything I hit seems to be going in, I'm delighted."
Croatia coach Slaven Bilic saw his team labour to a 1-0 win over Belarus on Saturday but rued the sending off of defender Vedran Corluka, now out of Wednesday's match.
"We were planning specific set plays with him for the England match," Bilic told Croatian media.
"He's strong and means a lot to the team, so we will have to think of something else to find an alternative for his absence at Wembley."
Corluka will be joined on the sidelines by his Spurs team mate Luka Modric, who broke his fibula in last week's Premier League win over Birmingham City.
Modric said a repeat of Croatia's 3-2 win over England at Wembley in 2007 would be "a perfect birthday present" when he turns 24 on Wednesday.
"My birthday will take a back seat in terms of significance because the match at Wembley is the main event of the day and I will only celebrate if we win or at least get a draw," Modric told daily Vecernji List.
