diff --git a/src/systems/diffeqs/abstractodesystem.jl b/src/systems/diffeqs/abstractodesystem.jl
index f32af70091..352362f14d 100644
--- a/src/systems/diffeqs/abstractodesystem.jl
+++ b/src/systems/diffeqs/abstractodesystem.jl
@@ -768,12 +768,14 @@ end
 struct GetUpdatedMTKParameters{G, S}
     # `getu` functor which gets parameters that are unknowns during initialization
     getpunknowns::G
-    # `setu_oop` functor which returns a modified MTKParameters using those parameters
+    # `setu` functor which returns a modified MTKParameters using those parameters
     setpunknowns::S
 end
 
 function (f::GetUpdatedMTKParameters)(prob, initializesol)
-    f.setpunknowns(prob, f.getpunknowns(initializesol))
+    mtkp = copy(parameter_values(prob))
+    f.setpunknowns(mtkp, f.getpunknowns(initializesol))
+    mtkp
 end
 
 function get_temporary_value(p)
@@ -836,16 +838,13 @@ function process_DEProblem(constructor, sys::AbstractODESystem, u0map, parammap;
         end
     end
     defs = defaults(sys)
-    missingpars = [p
+    guesses = merge(ModelingToolkit.guesses(sys), isempty(guesses) ? Dict() : todict(guesses))
+    solvablepars = [p
                    for p in parameters(sys)
-                   if (parammap !== SciMLBase.NullParameters() &&
-                       get(parammap, p, nothing) === missing) ||
-                      ((parammap isa SciMLBase.NullParameters ||
-                        get(parammap, p, nothing) !== missing) &&
-                       get(defs, p, nothing) === missing)]
+                   if is_parameter_solvable(p, parammap, defs, guesses)]
     # ModelingToolkit.get_tearing_state(sys) !== nothing => Requires structural_simplify first
     if sys isa ODESystem && build_initializeprob &&
-       (((implicit_dae || !isempty(missingvars) || !isempty(missingpars)) &&
+       (((implicit_dae || !isempty(missingvars) || !isempty(solvablepars)) &&
          ModelingToolkit.get_tearing_state(sys) !== nothing) ||
         !isempty(initialization_equations(sys))) && t !== nothing
         if eltype(u0map) <: Number
@@ -861,9 +860,8 @@ function process_DEProblem(constructor, sys::AbstractODESystem, u0map, parammap;
         punknowns = [p
                      for p in all_variable_symbols(initializeprob) if is_parameter(sys, p)]
         getpunknowns = getu(initializeprob, punknowns)
-        setpunknowns = setp_oop(sys, punknowns)
+        setpunknowns = setp(sys, punknowns)
         initializeprobpmap = GetUpdatedMTKParameters(getpunknowns, setpunknowns)
-        # TODO: Initializeprobpmap when setp_oop is a thing
 
         zerovars = Dict(setdiff(unknowns(sys), keys(defaults(sys))) .=> 0.0)
         if parammap isa SciMLBase.NullParameters