The 23-year-old came off the bench to rescue the Three Lions from the jaws of defeat against Sweden, providing the equaliser and then teeing up Danny Welbeck to score a priceless winner and subsequently hand them a great chance of progressing from Group D.
However Walcott, who missed several games for Arsenal last season with hamstring trouble, could not complete Sunday's session at the team's training base in Krakow, and is now a serious doubt for the clash with the co-hosts.
"Unfortunately he had a slight setback in training," Hodgson told BBC Radio 5 Live.
"He felt the hamstring that kept him out for so long at the end of the season and which we were slightly concerned about when he came to us.
"We have been lucky nothing has occurred with that but in a little training session today we had to take the precaution of taking him from the field because he felt some tightness there.
"Whether he is a real contender for a starting position will depend upon what the medical people can tell me and how much guarantee they can give me that if I do start him he won't be leaving the field after five minutes, forcing us to have 13 men rather than the 14 I would rather count on."
Meanwhile, the England boss showered praise on left-back Ashley Cole, who will make his 21st appearance at a major international tournament, breaking the record jointly held by Peter Shilton and David Beckham.
"He misses very few games, either for his national team or club team, and always plays to a very high level," Hodgson .
"It is great to hear that he has this record to beat when he plays against the Ukraine. I am delighted for him.
"It would be wrong to take him for granted. We are very much aware of his qualities and how much we need him down that left side.
"Because he is a quiet man, maybe for a lot of people he does slip under the radar.
"I know he is most comfortable when he lets his football do the talking."Hodgson will be anxious to have Walcott available to face Ukraine at the Donbass Arena, although he will be buoyed to have key man Wayne Rooney back from suspension.