Central defender Howedes, who scored in the quarter-final second leg against Inter Milan, will know his fate as late as Tuesday morning, coach Ralf Rangnick told reporters.
"The earliest we will decide on whether he will play or not is after our last training or even on the matchday itself," said Rangnick, who steered his team to their first ever Champions League semi-final days after taking over from Felix Magath last month.
Howedes, who has helped Schalke to a 100 percent home record this season, picked up an abdominal muscle injury during their 1-0 defeat by Kaiserslautern on Saturday.
Rangnick said while Schalke were rank outsiders, he felt confident a spot in the final was not impossible for the team lying 10th in the Bundesliga.
"We know we are outsiders but we were also outsiders against Inter in the quarter-finals. Despite being in this situation, which in a way makes us feel good, we want to get the best out of it. Obviously our aim is to make the final."
MINOR INJURIES
Rangnick can also count on midfielders Peer Kluge and Kyriakos Papadopoulos, who are fit after minor injuries.
"Kyriakos will be playing and for Peer there is a good chance to make the squad."
Rangnick said teamwork would be key to their success against a team who have not conceded a goal in their Champions League away games this season.
"Manchester are more organised and more compact than Inter, he said. "We know our opponents don't just have (Javier) Hernandez and (Wayne) Rooney in attack but several more good players.
"We have to operate well as a team because you cannot expect to get an easier opponent when you advance from the quarter-finals to the semi-finals."
"But a younger, hopefully hungrier team plays a very experienced and hugely successful team," said Rangnick when asked about his team's chances.
"We have to show the fans and our opponents how highly we rate this situation."
