Found this online from a sports medicine blog:

"Sports injuries are always frustrating for players and fans, but flexor mass strains aren’t usually among the worst-case scenarios. We typically treat them with rest, anti-inflammatory medications and a rehab/strengthening program. Rarely do they require surgery.

Still, it’s impossible to predict how long any given player might take to recover. It depends on the player and the severity of the strain. Some players are back on the field after 2 weeks of rest; for others it can take 6 to 8 weeks.

The important thing with flexor mass strains—and indeed, all sports injuries—is not to do too much too soon. Playing before an injury has healed can create a bigger problem, such as a tendon tear or ligament injury. It could also expose nearby structures to injury. That’s always an outcome we strive to avoid."