As noted, Sami Zayn defeated Braun Strowman in the Falls Count Anywhere main event of last night's RAW to earn Braun's spot in the men's Money In the Bank Ladder Match at the May 19 WWE Money In the Bank pay-per-view.
Strowman is currently dealing with an injury, according to Dave Meltzer of Wrestling Observer Radio, but that is not why he was pulled from the match. There's no word yet on injury details, but Strowman could be working through some sort of biceps or triceps injury as he was wearing a black band on his upper right arm.
Regarding Strowman losing his Money In the Bank spot to Zayn, Meltzer reported that the removal was not due to the injury and will be a part of a longer storyline that will play out over the next month or so. Strowman is working hurt, but the injury is not bad enough that he needs to take any time off right now.
There's been a lot of speculation on Strowman's WWE future as of late. He reportedly received heat from WWE officials after winning the 6th Annual Andre the Giant Memorial Battle Royal at WrestleMania 35. The end of the match was said to be a mess as Strowman forgot some of his spots. It was reported in late April that the botched spots are why Strowman hasn't had much of a push since winning the Battle Royal.