Clark was designated for assignment on Saturday to make room for pitcher Freddy Garcia on the 25- and 40-man rosters.
Clark made his major league debut May 1 and allowed three runs and three hits in 1 2/3 innings.
Clark (UMBC) was 1-2 with a 4.56 ERA in five starts at Triple-A Norfolk. The Tides rotation is full with Steve Johnson, Jair Jurrjens, Zach Britton, Jake Arrieta and Josh Stinson.
Update: Clark will work with Hall of Famer Phil Niekro to develop a knuckleball.
Niekro will be at Bowie on Thursday to work with Clark and Eddie Gamboa, who's using the pitch now and is 0-3 with a 3.81 ERA in six starts with the Baysox.
"Zach is an experienced, capable pitcher and has always had a good knuckleball, which he will now throw in competition," said executive vice president Dan Duquette.