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."
Capello, who prefers to play a big man like Heskey alongside Rooney, said he was pleased with the way Heskey performed in the first half against Slovenia.
England lead Group Six with 21 points from their seven matches, with Croatia second (17 points from eight games) and Ukraine third (14 from seven).
England, 4-1 winners in Croatia last year, are keen to beat them again and avenge the 3-2 loss at Wembley two years ago which cost them a place in Euro 2008.
Croatia are likely to be involved in a battle for the second place playoff spot with Ukraine, but would still have an outside chance of automatic qualification if they won on Wednesday and other results went their way.
CORLUKA OUT
The winners of the nine European groups qualify directly for South Africa and the best eight runners-up play off for four more places.
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.
Ivan Rakitic proved a worthy replacement for Modric, scoring the winner against Belarus, and Bilic is likely to field his preferred 4-4-2 formation with a diamond-shaped midfield.
Probable teams:
England:
